16. 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.

求教：此题，我认为A和B都可以，不知道大家的意见？请指教

A应该不可以。Without such a stable and moderate axis tilt, a planet’s

climate is too extreme and unstable to support life这句话表明：有一个合

适的moon（A）只是星球有生命（B）的一个必要条件，而不是充分条件。

A是说有A即有B，从必要条件得不出这个。（这需要是充分条件才能得出）

B是说非A即非B，符合必要条件。