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.

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


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

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

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

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

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.

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

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.

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.

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

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.
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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!
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