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.

正确答案是B

火星没有大月亮,推出火星没生命,是原命题

A 中,火星有了大月亮,推出火星有生命,是否命题

原命题对,否命题必然错

逆否命题才对,既火星有生命,推出必然有大月亮

谁教你说原命题成立 否命题就一定不成立的？

只是说否命题不一定成立

原文的是这样的 1 有倾斜--〉一定有大卫星（等价于没大卫星--〉没倾斜） 2没倾斜--〉没生命

所以 没大卫星--〉没倾斜--〉没生命

B

稍微有了一点头绪。。。可是为什么是有倾斜才有月亮呢？文中的意思应该是有月亮所以才有倾斜吧。（1）

有月亮，有倾斜。推出，没倾斜，没月亮。（2）没倾斜，没生命。所以决定一切的是倾斜度，而不是月亮。所以A错。

这样理解不知道对不对呢？

不对 就算条件一你看反了 条件二也不能等价于 有倾斜就有生命 因为否命题不一定成立 条件二的等价命题是 有生命则一定有倾斜

如果把A反过来说就对了 火星有生命--〉有倾斜--〉有月亮

汗

原命题成立 否命题就一定不成立      这是客观事实   没啥可争的