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When asteroids collide, some collisions cause an asteroid to spin faster; others slow it down. If asteroids are all monoliths-single rocks-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 discovery of which of the following would call into question the conclusion mentioned in line 16 ?
- AAn asteroid 100 meters in diameter rotating at a rate of once per week
- BAn asteroid 150 meters in diameter rotating at a rate of 20 times per hour
- CAn asteroid 250 meters in diameter rotating at a rate of once per week
- DAn asteroid 500 meters in diameter rotating at a rate of once per hour
- EAn asteroid 1,000 meters in diameter rotating at a rate of once every 24 hours
- 怎么都想不明白为什么这道题选D，文章不是说那些直径大于200的rubble piles 转的更快，once a few hour。选D不就是增强而不是削弱了吗？
- . 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.
- The author of the passage mentions "escape velocity" (see line 22) in order to help explain which of the following?
- AThe tendency for asteroids to become smaller rather than larger over time
- BThe speed with which impact fragments reassemble when they do not escape an asteroid’s gravitational attraction after a collision
- CThe frequency with which collisions among asteroids occur
- DThe rotation rates of asteroids smaller than 200 meters in diameter
- EThe tendency for large asteroids to persist after collisions
- 为什么B是错的 ？
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发表于 2017-7-6 17:32:01
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楼主
，谢谢小仙女
