单项选择题

  Line      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
    (5)    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
    (10)    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
    (15)    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
    (20)    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
    (25)    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