--She was in class on Friday afternoon, but no one has seen her ever since. --She ____an accident. A. must have B. might have had C. could have had D. should have had 答案:B下列所描述的运动中,可能正确的有:
A. A.速度变化很大,加速度很小 B.速度变化方向为正,加速度方向为负
C.速度变化越来越快,加速度越来越小 D.速度越来越大,加速度越来越小