Some say that an asteroid colliding with Earth was responsible for the

It is likely that an asteroid collision did occur and caused the death of the vast majority of dinosaurs.

However, new evidence suggests that perhaps many dinosaurs lived far enough that they were not directly killed by the impact or by the drastic change in temperature when dust blocked the sun.

Conclusion: Some other force must have been responsible for the death of these remaining dinosaurs.

The author says that it is likely that a meteor killed majority of dinosaurs but that many dinosaurs lived far enough to not get a direct hit of the impact or the temp change. So, some other "force" (not the asteroid collision) must have been responsible for wiping out the rest.

What is the assumption here?
That some other aspect of asteroid collision which would impact dinosaurs all across the globe is not responsible for extinction.

A. The water supply used by the dinosaurs who survived the asteroid collision and subsequent change in global temperature had not become contaminated by lethal fallout from the collision.

Correct. This gives you a different aspect (water contamination) of asteroid collision that could have impacted far away dinosaurs. This is an assumption.

B. A second asteroid impact or some other cosmic event is likely to have killed off the dinosaurs that survived the asteroid impact and global temperature change.

No assumption about a second asteroid or cosmic event. The author just says "other force". It may not be another cosmic event.

C. At least a few dinosaurs continued to exist for some years after the asteroid collided with Earth.

The immediate problem with this option is "some years". The author doesn't need to assume that some dinosaurs existed for some years. She just says that all were not killed by direct impact. Even if some survived for 2 months after impact, it is enough for her argument. We cannot assume anything about this timeline.

D. There is another existing theory that does a better job of explaining the extinction of the dinosaurs than the theory involving an asteroid collision with Earth.

No such assumption. She says that it is likely that asteroid collision killed majority of dinosaurs. She is looking for an additional explanation to complete the event.

E. The change in temperature caused when dust clouds from the asteroid impact blocked the sun could not have lasted for several years.

No such assumption. We don't know how many years or months or days it lasted and how that impacted dinosaurs in that area and elsewhere.

Answer (A)

New evidence suggests that perhaps that dinosaurs lived far enough from the point of collision that they were not directly killed by the
1)impact or
2)by the drastic change in temperature when immense dust clouds created by the impact blocked the sun.

Conclusion:
===>Some other force must have been responsible for the death of these remaining dinosaurs.

The author says that "some other force" must have been responsible for the extinction of the remaining dinosaurs. It excludes the possibility that the FIRST asteroid had other effects. So the answer choice A is correct because exclude (partially) this possibility. This kind of assumptions "fill" a gap of the reasoning.

The other answer choices:

c) At least a few dinosaurs continued to exist for some years after the asteroid collided with Earth.
Even if this part "refers" to something that is in the argument, the presence of dinosaurs is NOT the argument of the author; so this answer choice is not necessary to reach the conclusion.
d) There is another existing theory that does a better job of explaining the extinction of the dinosaurs than the theory involving an asteroid collision with Earth.
The existence of another theory is not necessary to the argument.
e) The change in temperature caused when dust clouds from the asteroid impact blocked the sun could not have lasted for several years.
The duration of the dust clouds is not relevant in the reasoning.

B IMO is the second contender
b) A second asteroid impact or some other cosmic event is likely to have killed off the dinosaurs that survived the asteroid impact and global temperature change.
This answer says that a cosmic event is likely to have killed the remaining dinosaurs.
The author says:
"Some other force must have been responsible for the death of these remaining dinosaurs."
And answer choice B says that the responsible force is(probably) a cosmic event: this can't be an assumption. The argument is still valid even if a cosmic event is NOT "likely to have killed off the dinosaurs (...)"(B negated); and maybe volcano eruptions are responsible, for example.

CONCLUSION:
Someotherforce must have been responsible for the death of these remaining dinosaurs.

a) The water supply used by the dinosaurs who survived the asteroid collision and subsequent change in global temperature had not become contaminated by lethal fallout from the collision.==>THIS SHOULD BE THE ASSUMPTION AS IF WE NEGATE THIS:
The water supply used by the dinosaurs who survived the asteroid collision and subsequent change in global temperatureHAD become contaminated by lethal fallout from the collision.
THEN IN THIS CASE IT SAYS IT WAS THE EFFECT OF ASTEROID COLLISION ONLY WHICH LEAD TO DEATH OF DINOSAURS.

b) A second asteroid impact or some other cosmic event is likely to have killed off the dinosaurs that survived the asteroid impact and global temperature change.
IT IS TOO SPECIFIC TO SAY THAT SECOND ASTEROID IMPACT OR COSMIC EVEN LED TO DEVASTATION....this cant be assumption

c) At least a few dinosaurs continued to exist for some years after the asteroid collided with Earth.
not necessary it could also possible that during asteroid impact some other factor also came in to play leading to death of dinosaurs.hence this cant be assumption.

d) There is another existing theory that does a better job of explaining the extinction of the dinosaurs than the theory involving an asteroid collision with Earth.
this is out of scope we are not concerned whether there is some other theory or not we are just concerned that some other factor led to death of dinosaurs
e) The change in temperature caused when dust clouds from the asteroid impact blocked the sun could not have lasted for several years.
the duration of blocked sun doesnt affect the conclusion.

Have some doubts with A.
it Says : a) The water supply used by the dinosaurs who survived the asteroid collision and subsequent change in global temperature had not become contaminated by lethal fallout from the collision.

Lets Negate A : The water supply.....got contaminated by lethal fallout from the collision.
Ie lets assume water was contaminated, but still contamination does not mean death of dinosaurs.
It might be possible that water got contaminated and dinosaurs got ill because of the contaminated water but didn't die.

So, here negation of A still does not break the argument...!!!

please comment...

The argument already states very clearly that some Dinosaurs did survive the aftermath of the collision.The author concludes that some other force must have been responsible for their deaths. Hence "A" suggests a ripple effect of the collision did not occur. So only if "A is true the author's conclusion becomes valid. "C" goes against the fact because it is clearly suggested in the argument that some Dinosaurs did survive.

How can A be an assumption? it is doing the job of strengthening.

Moreover, the option A is supporting the new evidence found of survival of dinosaurs after collision.

Only thing seems to be ok is E. but not A

"E" is an additional fact. It is not an assumption. Moreover, the term "several years" is vague. It does not indicate the exact number of years. On the other hand "Water contamination" is the direct effect. Taking the facts to be true and if there was no immediate water contamination as stated in "A", then the conclusion becomes credible that the remaining dinosaurs did not die as a consequence of the asteroid collision.

"B" suggests that there might have been a second asteroid or some other cosmic event which killed the remaining dinosaurs. The author has clearly mentioned in the argument that the dinosaurs were not killed directly by the impact or the temperature change which occurred after the collision with the first asteroid. So "some other force" indicates something completely different. " B" reiterates on a similar force. Hence that cannot be the assumption made by the author. "A" suggests an effect which is neither caused by the direct impact nor the temperature change. Hence "A" is the necessary assumption out here.

Question Type: Inference

(A) No comparably catastrophic asteroid collision with Earth has occurred since the one that killed the vast majority of dinosaurs. - Incorrect. Very Extreme.

(B) Some members of each dinosaur species were killed either directly or indirectly by the impact of the asteroid collision with Earth. - Incorrect. We cannot conclude that some members of each dinosaur species were killed. May be an entire specie was separated from the group that faced an asteroid impact.

(C) All the members of at least one dinosaur species were killed either directly or indirectly by the impact of the asteroid collision with Earth. - Incorrect. Extreme.

(D) If the majority of dinosaurs were killed by an asteroid colliding with Earth, some dinosaurs lived at great distances from each other at the time of the collision. - Correct.

(E) It is possible to determine a close estimate of the number of dinosaurs that were killed, either directly or indirectly, by the asteroid collision with Earth - Incorrect. Out of scope. Estimating the number of deaths is not discussed at all in the argument.

Answer: D

While it is obvious that A is the correct answer choice, it is important to understand why C is incorrect. If you take a look again and pay attention to the flow of the argument, you'll notice that the fact that several dinosaurs survived the impact and the subsequent temperature change is a premise, not an assumption. Without the surviving dinosaurs, there is no argument. Also, this fact is not even necessarily true - notice the word "perhaps" in the sentence.

Another way to look at this fact is to regard it as a hypothetical situation: if there were surviving dinosaurs, they must have subsequently been killed by some other force, and if there were not any, ah well, there is nothing left to say.

Hi Expert GMATNinja, @e-gmat and others

I do not find Choice A as the correct answer choice. The argument would break if on negating the correct answer choice we can show:

Either asteroid collision directly caused the extinction
Asteroid collision led to a change in temperature that caused the extinction

Now choice A says that asteroid collision contaminated the water and it could be the dinosaurs drank this water(note that even this is not mentioned that dinosaurs happened to drink it). Even if we assume that they did, still this is an indirect method of the extinction and hence does not break the argument!

Please advise.

Hi Jay

The argument can be broken down as follows:

Conclusion: "Some other force must have been responsible for the death of these remaining dinosaurs."

Premises:

i) It is indeed likely that such a collision did occur and that it caused the death of the vast majority of dinosaurs.
ii) Perhaps tens of thousands of dinosaurs lived far enough from the point of collision that they were not directly killed by the impact or by the drastic change in temperature when immense dust clouds created by the impact blocked the sun.

Option A: The water supply used by the dinosaurs who survived the asteroid collision and subsequent change in global temperature had not become contaminated by lethal fallout from the collision.

If we negate option A: The water supply used by the dinosaurs who survived the asteroid collision and subsequent change in global temperature had become contaminated by lethal fallout from the collision.

If their water supply was contaminated (it is ok to assume that the dinosaurs did indeed consume said water supply, otherwise they would have died of thirst anyway) then it is likely the remaining dinosaurs too died due to the fallout of the asteroid collision and not due to any other force. This breaks the conclusion that some other force (ie; other than the asteroid collision) caused the death of the remaining dinosaurs.

Hope this helps.

I don't understand why A is the answer.

Conclusion - Other factors were responsible for the death of dinosaurs.

On negation- A becomes "The water supply used by the dinosaurs who survived the asteroid collision and subsequent change in global temperature had become contaminated by lethal fallout from the collision."

Contaminated water must be lethal. Therefore, it supports the conclusion. Shouldn't negating the assumption break down the conclusion rather than support it?

Or have I misunderstood negation?

GMATNinja Please help.

Hi,
The water getting contaminated by lethal fallout from the collision indicates that the contamination was an after effect of the asteroid collision. Therefore, it was not any other force but the collision itself responsible for the extinction of the remaining dinosaurs as well and hence it breaks the conclusion.

GMATNinja would love to hear your thoughts on this.

I prephrased that none of the factors, directly or indirectly linked with the asteroid collision caused the extinction, along this line (A) is correct, however, I don't see anything wrong with (C) which says that some dinosaurs ended up surviving.

If we logically negate this, we get;
"None of dinosaurs survived after the asteroid collision"...from this we get that since none survived, other factor (including related to asteroids OR not related to asteroids) eliminating the dinosaur could not have taken place. If any other factor did not take place, the conclusion breaks.

Is that too extreme? Please clarify

Other experts VeritasKarishma @EgmatVerbal CrackVerbal please help

The first thing that we should do when solving CR questions is to stop thinking way beyond the argument and inferring a lot.

Stick to the argument. Look for the answer option that fits our question requirement.

For how long the dinosaurs survived or how many dinosaurs survived is not really what we have to look for. We have to look for an option that states that it was not the collision or anything related to the collision that resulted in the extinction of the dinosaurs.

The author concludes that "some other force must have been responsible for the death of these remaining dinosaurs [that lived far away from the point of collision]."

Notice that the the author doesn't care about when these dinos died -- he/she only cares about what killed them.

So, which answer choice is assumed by the author in order to reach his/her conclusion? Here's (C):

Does this absolutely have to be true? Not necessarily -- as long as the dinos died from something not-asteroid-related, it's fine if two events happened at the same time. Maybe the asteroid hit on one side of the planet, and simultaneously a terrible disease broke out on the other side of the planet. If that's the case, then the author's argument holds, even if all of the dinos died at the same time.

(C) doesn't have to be true, so it's not an assumption made by the author.

Compare that to (A):

If the asteroid caused water to be lethally contaminated, and thus killed the dinos out of the asteroid's impact range, then the author's argument falls apart. The author's argument assumes that this didn't occur, so (A) is the correct answer.

I hope that helps!