OFFICIAL EXPLANATION

To attack this problem effectively, you must recognize the conditional indictors in each sentence: “only” in the second sentence and “without” in the third sentence. Do not be intimidated by the science topic! As we will see, if you can understand the conditional form of the stimulus, the topic is of little concern. The two sentences produce the following conditional relationships, which can be linked: AS = angle stable, GI = gravitational influence of Earth’s large, nearby Moon, PCE = planet’s climate too extreme and unstable to support life.

Sentence 2: AS—>GI.
Sentence 3: ~PCE—>AS. Chain: ~PCE—>AS—>GI. Again, consider the answers that are most likely to appear in a problem like this: either the chain inference ~PCE—>GI, or the contrapositive of that chain inference, ~GI—>PCE.

Answer choice (A): This incorrect answer is the Mistaken Reversal of the chain inference. The diagram for this answer choice would be as follows, with the sub-M indicating Mars: GI—>~PCE.

Answer choice (B): This is the correct answer. As expected, this is the contrapositive of the chain inference.

Answer choice (C): This incorrect answer is the Mistaken Reversal of the third sentence, and is diagrammed as follows: AS—>~PCE.

Answer choice (D): This answer discusses issues that were not raised in the stimulus, and is thus incorrect.

Answer choice (E): The stimulus indicates that Earth’s large Moon has been necessary for the stable angle of Earth’s tilt, and this stable angle has been necessary for a climate that can support life. Mars, with many small moons, tilts at fluctuating angles and cannot support life. The key difference is that Earth’s Moon is large, and that creates a greater gravitational influence. It is possible therefore, that a planet with more than one moon could have a stable angle as long as at least one of the moons was of sufficient size (in Mars’ case, the stimulus indicates each moon is small). Thus, it is possible that a planet can have more than one moon and support life. This scenario is contrary to the answer choice, and thus this answer is incorrect.
_________________

The axis of Earth’s daily rotation is tilted with respect to the plane of its orbit at an angle of roughly 23 degrees.

Axis of Earth is tilted at roughly 23 degrees

That angle can be kept fairly stable only by the gravitational influence of Earth’s large, nearby Moon.

Only a large, nearby moon has the ability to keep the angle fairly stable.

Without such a stable and moderate axis tilt, a planet’s climate is too extreme and unstable to support life.

Without this moderate tilt (23 degrees) which is stable, planet's climate is too extreme to support life.

Mars, for example, has only very small moons, tilts at wildly fluctuating angles, and cannot support life.

Mars, which has small moons, tilts at wildly fluctuating angles (so the angle is neither moderate nor stable). It cannot support life.

What must be true?

(A) If Mars had a sufficiently large nearby moon, Mars would be able to support life.

No. If Mars had a sufficiently large Moon nearby, it might have been able to keep the angle stable. Would the angle be moderate? We don't know. Would Mars have had a million other things needed to support life? We don't know.
The statement is a hypothetical conditional. If A were true, B would have happened.
But we cannot say that B would have happened.

(B) If Earth’s Moon were to leave Earth’s orbit, Earth’s climate would be unable to support life.

Correct. We are given that only a large nearby moon can keep the angle stable. Without this moderate, stable tilt, Earth's climate would not be able to support life. If the Moon were not there, the angle would not be stable and hence no life.

(C) Any planet with a stable, moderate axis tilt can support life.

Not correct. We need a million other things to go right too. A stable, moderate axis tilt is necessary, not sufficient.

(D) Gravitational influences other than moons have little or no effect on the magnitude of the tilt angle of either Earth’s or Mars’s axis.

We don't know what has effect on the magnitude of the tilt angle. We just know that Moon is able to keep the angle stable.

(E) No planet that has more than one moon can support life.

Not given.

Answer (B)
_________________

I find 'B' the right option because, the passage is talking about earth's moon and not any other factor that might be involved in maintaining the stability of the planet, and it has been further supported by another example of the moon, of a different planet. The answer to this question should be the one which considers just this factor. So, if the Earth's moon leaves the orbit, then the stability of the planet would be disturbed.

I pick option D.

a) The argument talks about Earth, so Mars is irrelevant
b) It is rather the gravitational influence of the moon that helps keep the angle of roughly 23 degrees. Even though it's true that Earth's climate will be unable to support life when the moon leaves, there's also no guarantee that Earth's climate will continue to support life if the moon is still around. Other factors such as global warming can eventually lead the climate to harm life even when the moon is still around.
c) we don't know whether a stable, moderate axis on other planets can support life, but we do know at least that it is keeping life that already exists on earth.
D) Correct. The argument says, "That angle can be kept fairly stable only by the gravitational influence of Earth’s large, nearby Moon."
E) We only know about earth. A sample can not represent the whole population. What works for earth may not necessarily works for other planets.

Don't agree.


The option is just talking about life, in case moon leaves. Its not talking about what happens otherwise.


Th option says "little or no effect on the magnitude ...." whereas argument says "That angle can be kept fairly stable ONLY by the gravitational influence ....", so "little effect" is out.

oh! I see it now! Option D also talks about Mars, which I've completely missed

yes, but even argument also talks about moons on Mars - "Mars, for example, has only very small moons,", but what I feel is its the auther's extreme language by using "only" in the sentence "That angle can be kept fairly stable only by the gravitational influence of Earth’s large, nearby Moon". That means that author is saying there is no other gravitational force responsible for this.

B for me too.
If we say A ==> B, (A leads to B) then the best test is to see whether
B still exists in the absence of A....
That way, we eliminate other causal factors.

Whats wong with C?

It is just a paraphased part of the stimulus:
The axis of Earth’s daily rotation is tilted with respect to the plane of its orbit at an angle of roughly 23 degrees. That angle can be kept fairly stable only by the gravitational influence of Earth’s large, nearby Moon. Without such a stable and moderate axis tilt, a planet’s climate is too extreme and unstable to support life. Mars, for example, has only very small moons, tilts at wildly fluctuating angles, and cannot support life.

Experts please explain.

Even I was looking for C as the answer but soon realized that statement mentions that Without such a stable and moderate axis tilt, a planet’s climate is too extreme and unstable to support life. but does not mentiones that this is not the only reason to support life so if any other planet is having stable and moderate axis tilt but does not have other supportive condition then it does not help.

I picked D after narrowing down to B & D. I agree that B should be the answer but I don't like it as the question didn't say anything about the earth's orbit. Doesn't sound like a good GMAT question.

the options B says "If Earth’s Moon were to leave Earth’s orbit", How can we assume that the Earth's moon is the large nearby moon ?? It could be any small moon right ?? Please explain !!!
_________________

The passage references "Earth’s large, nearby Moon." Sure, technically speaking, it is possible that the Earth has several smaller moons and the passage is only referring, in this case, to the "large, nearby" moon. But notice that a capital "M" is used in "Earth's Moon", and a lowercase "M" is used to refer to the "small moons" of Mars. A proper noun is used to refer to the Earth's Moon because we are referring to a specific astronomical body with a name (the Moon).

This same specific astronomical body is referenced again in choice (B). If the Earth had several smaller moons and the author wanted choice (B) to include any of those moons, it would have been: "If any of the Earth’s moon's (lowercase "m") were to leave Earth’s orbit, Earth’s climate would be unable to support life." More importantly, the other options can be eliminated using process of elimination.

Also, even though you shouldn't let reality get in the way of your GMAT, the GMAT won't ask you to swallow facts that contradict common knowledge... so don't lose too much sleep over the Earth's "smaller moons", unless you know something about astronomy that I don't!
_________________

Premise: The axis of Earth’s daily rotation is tilted with respect to the plane of its orbit at an angle of roughly 23 degrees.
Premise: That angle can be kept fairly stable only by the gravitational influence of Earth’s large, nearby Moon.
Conclusion: Without such a stable and moderate axis tilt (necessary condition), a planet’s climate is too extreme and unstable to support life (sufficient condition).
Additional Premise: Mars, for example, has only very small moons, tilts at wildly fluctuating angles, and cannot support life.

If the statements above are true, which one of the following must also be true on the basis of them?

The author argues that the angle of rotation (which is kept stable with the pull of the moon) is what make a planet’s climate able to support life. It then gives an example of Mars, which has wild tilts, and cannot support life. Overall, author concludes axis tilt necessary to support life.


(A) If Mars had a sufficiently large nearby moon, Mars would be able to support life.
To be correct, we’d have to assume that the “largeness” of the moon dictates ability to keep angle….and therefore support life.

(B) If Earth’s Moon were to leave Earth’s orbit, Earth’s climate would be unable to support life.
Correct. Argument says stable angle —> moon’s pull exists. No stable angle —> no life. (B) says no moon (no angle) —> no life.

(C) Any planet with a stable, moderate axis tilt can support life.
Too extreme. “Any” planet might have other problems that can’t support life.

(D) Gravitational influences other than moons have little or no effect on the magnitude of the tilt angle of either Earth’s or Mars’s axis.
We don’t care about influences other than moons….this is a support the information in the stimuli!

(E) No planet that has more than one moon can support life.
Well, this can’t be true. What if the planet has more than one moon but those moons happen to tilt the planet just enough to keep it stable?
_________________

Why not A as the answer?
I have tried eliminating it by the reasoning that since the stimulus specifically discusses about earth and mars is just an example.
Another method of reasoning is appreciated.

Posted from my mobile device

A is incorrect because in Mars case it has other moons too and it is nowhere stated in the passage that if Mars had a large moon nearby it would support life. That's what you are assuming but there's no support from the passage that says so.

In case of option C, It has been clearly stated that Earth can support life because it has a right tilt because of Moon's gravitational force. Hence, Earth cannot support life if there's no Moon.

Hi Bunuel, GMATNinja, VeritasKarishma

I am having trouble understanding the usage can vs would vs will in this question. Cons of reading too literally.
Could you please help me out here, please?

The way Bunuel has dealt with the question is efficient, but it leaves out will vs can discussion. So maybe it is not important, but I would like to understand if this something is worth fretting about.

The passage says:

That angle can be kept fairly stable only by the gravitational influence of Earth’s large, nearby Moon.

So can we infer the following:
If a large moon is present nearby, then the axis can be kept fairly stable.

Please notice that I am not saying:
If a large moon is present nearby, then the axis would/will be kept fairly stable.

According to you guys, is there a difference between the 2 statements above? The way I see how answer choices A and B are laid out, they are using would or will instead of can.

I have been taught that will = 100% certainty and can = ability to do something but not necessarily probable or guaranteed.

Please let me know what you guys think.

Thank you!

abhola - I am not sure I understand your doubt.
You are correct in saying that "can" shows ability and "will" shows certainty. "would" shows certainty in a hypothetical situation.
The use of "can" and "will/would" is appropriate in the argument and in the options (as explained above).
_________________

Hi VeritasKarishma,

My doubt is around the usage of 'only' and 'will vs can vs would'.

We are given the following statement in the passage:

That angle can be kept fairly stable only by the gravitational influence of Earth’s large, nearby Moon.

From this statement, could you please let me know which one of the below statements would be a valid inference?

Statement #1: If a large moon is present nearby, then the earth's axis can be kept fairly stable.

Here I am using 'can', and not 'would', meaning I am dealing with just the possibilities, not the certainty. So can I say #1, even though it looks like 'mistaken reversal' can be a valid inference?

Statement #2: If a large moon is present nearby, then the earth's axis would/will be kept fairly stable.

I think as you mentioned in the explanation of Choice A, this is most likely incorrect.

Thanks again!