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.
可否解释一下为什么不选C,题目不是说，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.
我理解是a planet是任意一个行星，然后举了Mars的例子，而答案局限于 Earth, 是不是缩小范围了呢？
这题是must be true题，因此答案可以是任何文中信息，关键看能不能从所给信息中推出。这题确实很典型。
That angle can be kept fairly stable only by the gravitational influence of Earth’s large, nearby Moon.
Only是必要条件提示词，也就是说only后面是必要条件。即23度→Moon
Without such a stable and moderate axis tilt, a planet’s climate is too extreme and unstable to support life.
我想问题的关键就是这句话了。没有23度，就没有life,也就是要有Life，必须有23度。（如果选C就理解成了：有23度就有Life）即life→23度
综上，life→23度→moon.其等价命题为：非moon→非life. 你看看是不是B。
充分条件和必要条件之间的区别。
Without such a stable and moderate axis tilt, a planet’s climate is too extreme and unstable to support life.
stable and moderate axis tilt是必要条件。
而C) Any planet with a stable, moderate axis tilt can support life.则把它说成是充分条件了。
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