John fell asleep ______ he was listening to the music. [ ]
A. after
B. before
C. while
答案:C
质量为3kg的小球,从40m高处由静止落下,空气阻力不计,g取10m/s2,则第2s内和第2s末重力做功的功率分别为多少( )
A.450w,600w
B.600w,600w
C.300w,600w
D.450w,450w
照明不足或天气影响,指挥人员看不清各工作地点或驾驶员看不见指挥时,()起吊工作。