答案:B
Questions 14-18
The building material needed of the Forbidden City were from ______.
用起重机将质量为m的重物匀速竖直提起一段距离,那么作用在物体上的各个力的做功情况是( )
A.重力做正功,拉力做负功
B.重力做正功,拉力做正功
C.重力做负功,拉力做负功
D.重力做负功,拉力做正功