HELP!
Can you explain me answers B and D?

B is correct because it addresses the issue of alarm saving lives.

D might serve as an explanation for why the company has caught less of porpoises, but does not address the saving their lives part.

Plz give more explanations.........still not clear why not D.......

Doesn't saved from being suffocated means aved from being killed.

I doubt if this is a text from GMAT.
I feel comfortable to assume that such a question is a creation of GMAT Tutors/ Private Organizations that prepare these questions for students benefit.

If this is infact a GMAT question, i expect it to be more precise as in option B.
There is an ambiguity here, other boats?? -- what are other boats?? -- 1)other boats of the same company or 2)other boats of different company that did not install the alarms??

Depending on the above, i would go with D if 1, i.e the other boats are mentioned as bpats of the same company, bcoz in such a case, B looses its importance as the average is already mentioned down to 1 from 8, so D is better as it supports the facts with its info.

But if other boats mean boats of other companies, I would go with B........ again if it says that other boats have no alarms installed on it.

This is my opinion, and if there are any experts around here, please thrash me if you feel my reasoning is wrong.

The right choice would be "B" and here's why:
When the option says "other boats" it definitely mean boats of other organizations which have not installed alarms, since the porpoises would scatter away and even if some of the "other" boats were of the same organization they would have alarms installed again scattering them away.. but now the ones which don't have such alarms would cause a problem.

The conclusion here is that the alarms are saving lives.
B - If the alarm is causing harbor porpoises to get killed by other boats, then ultimately the alarms are not saving lives.

D - If we negate this option, we cannot say for sure whether the alarms are saving lives or the population is less in general and therefore not many porpoises are getting killed.
does not have the impact that B has.

I'm 3 years late in replying you haha. oh wells.

Option D is: The alarms were installed at the time of year when harbor porpoises are most plentiful in the Massachusetts waters

So what if harbor porpoises are most plentiful? Maybe the number of boats increased too? Larger population/Much larger boat population also can achieve a drop from 8-->1

What B is saying that, yes, the company registered a drop from 8 --> 1 of those CAUGHT IN NETS, but what about those disoriented porpoises that just swim straight into a boat and gets killed by other means? maybe they swam into the boats rotor which sliced them up? so the company registers a drop in those caught by nets but doesn't consider other possible means of death

To prevent harbor porpoises from getting tangled in its nets and suffocating, a fishing company installed acoustic alarms on all its boats that fish in waters off Massachusetts. The sound emitted temporarily disorients the porpoises and frightens them away. Since the installation of the alarms, the average number of porpoises caught in the company's nets has dropped from eight to one per month. The alarms, therefore, are saving the lives of harbor porpoises in those waters.

Type - Assumption
-Fishing company has installed alarms on boats
-Sound emitted temporarily disorients the porpoises and frightens them
- Monthly Average number of porpoises caught in nets has dropped from 8 to 1 .
Conclusion - Alarms are saving the lives of porpoises

Which of the following is an assumption on which the argument depends?

A. The use of acoustic alarms increases the number of commercial fish caught by the fishing company's boats. Irrelevant

B. When disoriented, harbor porpoises are not significantly more likely to be killed by other boats. Correct answer - if this is true , the alarms are not saving the lives of porpoises

C. Environmentalists advocate the use of acoustic alarms as a means of protecting the harbor porpoise population. Irrelevant - the opinion of Environmentalists does not matter

D. The alarms were installed at the time of year when harbor porpoises are most plentiful in the Massachusetts waters. Incorrect - the quantity does not matter as long as the alarms are saving the lives the porpoises .Also , even if we negate this , the alarms were installed at the time of year when porpoise population was below the peak , the alarms might still help in saving lives .

E. The cost of installing acoustic alarms on fishing boats is less than the cost of repairing nets damaged by harbor porpoises. Irrelevant

Answer B
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D also contradicts the question stem's use of "average" when describing the number saved.

Negation of B directly destroys the cocnlusion. But how to eliminate option D?

The conclusion is based on the premise that propoises caught has dropped from 8 to 1. What if the alarms were insatalled when the propoises were not plentiful and the number drop is due to less propoises.

Please explain why D is wrong?

Could someone help to explain B is an assumption we have to consider? Why is the 'disoriented' in option B mean? This confused me. I do understand if the purpoises are not more likely to be killed by other boats, then we can confirm that the alarms actually do save the lives of harbor purposes. Please help to clarify this doubt. Thanks.

The support is ~"the company's plan (alarms) helped to reduce the number of porpoises killed by its (the company's) boats". The conclusion is that the company's plan is "saving the lives of harbor porpoises in those waters".

There is a clear "gap" here between the number of porpoises killed by the company's boats and the number of porpoises dying "in those waters": what if the company's plan causes something to happen that makes the porpoises significantly more likely to be killed by some other means? In that case, even if the company's own boats don't kill as many porpoises as they used to, the company's plan will end up leading to the deaths of many more porpoises. That's what option B addresses. It says that the probability that harbor porpoises will be killed in some other way is not high, and therefore the net impact of the company's plan is likely to be positive.

The "disoriented" bit just describes the effect on the porpoises of the alarms that the company installed on its boats ("the sound emitted temporarily disorients the porpoises").
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The question asks "Which [answer choice] is an assumption on which the argument depends." So, we are looking for an answer choice that MUST be true in order for the author to reach his/her conclusion.

The conclusion is that "The alarms... are saving the lives of harbor porpoises in those waters."

Evidence for this conclusion is that "since the installation of the alarms, the average number of porpoises caught in the company's nets has dropped from eight to one per month."

With this in mind, take another look at (D):

Does this absolutely have to be true in order for the author to reach his/her conclusion?

Think through the argument if the author does not assume that the nets were installed during the time of year when porpoises are most plentiful. Perhaps the alarms were installed when the population was low, and then a bunch of porpoises arrived in the months after. This would strengthen the evidence in the passage, because the number of porpoises caught in nets has decreased despite the overall population increasing. Because the argument does not fall apart if (D) is not assumed, (D) is not an argument on which the argument depends.

Note: this hypothetical situation is sufficient to eliminate (D), but you could think of many additional cases in which this answer choice impacts the conclusion in different ways.

Let's say that the alarms were installed at the time of year when porpoises are most plentiful -- what does that do to the conclusion? It depends on several additional factors. Maybe a huge majority of the porpoises left the Massachusetts waters immediately after the alarms were installed, so the number getting caught in the company's nets would decrease with or without the influence of the alarms. This would weaken the author's conclusion. Maybe the large number of porpoises did stick around -- if this were the case, the evidence in the passage would not be affected.

Overall, we cannot say how the information in (D) impacts the author's conclusion, so it is definitely not an assumption on which the argument depends.

I hope that helps!
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Type: Find the assumption

Conclusion: The alarms, therefore, are saving the lives of harbor porpoises in those waters.

(A) The use of acoustic alarms increases the number of commercial fish caught by the fishing company's boats. - irrelevant to the conclusion.

(B) When disoriented, harbor porpoises are not significantly more likely to be killed by other boats. - if this were true, then we can infer that the disorientation caused by the ''alarms'' do not endanger the porpoises. However, if the disorientation caused by these alarms puts these porpoises at direct risk, then we no longer can support the conclusion drawn. Hence, we need to assume (B) in order for the conclusion to hold. Therefore, (B) is the right answer choice.

(C) Environmentalists advocate the use of acoustic alarms as a means of protecting the harbor porpoise population. - has no bearing on the conclusion.

(D) The alarms were installed at the time of year when harbor porpoises are most plentiful in the Massachusetts waters. - irrelevant.

(E) The cost of installing acoustic alarms on fishing boats is less than the cost of repairing nets damaged by harbor porpoises. - irrelevant.

GMAT experts.. Please help.. How B is correct and not D???

A - it is out of scope assumption that doesn't influence the argument, and it has nothing to deal with the protection of porpoises.

B - This answer choice correctly provides the assumption that shows the main purpose on which the conclusion of such an argument is based. - Correct Answer

C- It's out of scope, we have nothing to do with environmentalists. Who cares what they think.

D- Also wrong, since it talks when the alarms are installed rather than why, and what it's the main purpose.

E - Out of scope as well, as this argument has nothing to deal with the cost of the alarm installation.

The main concern about the porpoises is to protect them (May be they are endangered or some other concerns which is out of scope of dicussion)

Now between (B) and (D)

(B) If yes alarm disorients harbor porpoises and they move far away they are saved , if no then the objective of saving the harbor porpoises.

Thus, Option (B) must be a necessary condition and is necessary to achieve the conclusion highlighted in blue.

(D) If yes alarms were installed when fish population is high and yet they are not trapped in fishing net - Its is successful

but if they were not installed at a time when harbor porpoises population was not high then it will not be correct to reach the conclusion...

Hence (B) looks a better Answer compared to (D)
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Conclusion:
The alarms, therefore, are saving the lives of harbor porpoises in those waters.

Just negate option B
(B) When disoriented, harbor porpoises are not significantly more likely to be killed by other boats.

Totally breaks the conclusion

Hence option (B) is correct