When asteroids collide, some collisions cause an asteroid to spin faster; others slow it down. If asteroids are all monoliths—singlerocks—undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with statistical “tails” of very fast and very slow rotators. If asteroids are rubble piles, however, the tail representing the very fast rotators would be missing,because any loose aggregate spinning faster than once every few hours(depending on the asteroid’s bulk density) would fly apart. Researchers have discovered that all but five observed asteroids obey a strict limit on rate of rotation.The exceptions are all smaller than 200 meters in diameter, with an abrupt cutoff for asteroids larger than that.
The evident conclusion—that asteroids larger than 200 meters across are multicomponent structures or rubble piles—agrees with recent computer modeling of collisions, which also finds a transition at that diameter. A collision can blast a large asteroid to bits, but after the collision those bits will usually move slower than their mutual escape velocity. Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile. Because collisions among asteroids are relatively frequent, most large bodies have already suffered this fate. Conversely, most small asteroids should be monolithic, because impact fragments easily escape their feeble gravity.

The author of the passage mentions “escape velocity” （see highlighted text） in order to help explain which of the following?

The tendency for asteroids to become smaller rather than larger over time

The speed with which impact fragments reassemble when they do not escape an asteroid’s gravitational attraction after a collision

The frequency with which collisions among asteroids occur

The rotation rates of asteroids smaller than 200 meters in diameter

The tendency for large asteroids to persist after collisions

##### 考题讲解

A选项：小行星变小而不是变大的趋势：无关。

B选项：碰撞之后，碎片聚在一起速度：逃跑速度解释了为什么碎片聚在一起，但没有解释聚在一起的速度是多少。

C选项：行星碰撞的频率：无关。

D选项：

E选项：正确。