Mary is____a ballet. [ ]
A.looking
B.seeing
C.looking at
D.watching
答案:D
升旗时,缓缓向下拉绳子,旗就会徐徐向升,这是由于旗杆顶部有一个______(选填“动滑轮”或“定滑轮”),它能改变力的______,但不省力.