题目

Try recording your voice in English--this may help you to see__________ you could improve. A. that              B. when               C. how                 D. where 答案：D起重机用2000N的拉力拉着物体以1m/s的速度匀速上升，如果它吊着物体以2m/s的速度匀速下降，则此时起重机对物体的拉力是    N．