题目

 The thief froze in fear _____ he heard footsteps _____ .   A. while; approaching            B. while; approach   C. when; being approached        D. when; approaching 答案：D在水平桌面上叠放着木块P和Q,用水平力F推Q,使P、Q两木块一起沿水平桌面匀速滑动,如图所示,以下说法中正确的是（ ）A.P受三个力,Q受六个力 B.P受四个力,Q受六个力C.P受两个力,Q受五个力 D.P受两个力,Q受四个力