——Mary stays alone in the classroom and looks sad.

——________ to her?

A．What happens

B．What is happened

C．What is happening

D．What was happening

用起重机把质量为2.0×10³kg的物体匀速地提高9m。重力加速度g取10m/s²。试求：（1）钢绳的拉力做了多少功？

（2）重力做了多少功？

（3）若钢绳拉力改为2.4×10⁴N，起重机把物体由静止开始匀加速提高9m，则在此过程中钢绳拉力做功的功率为多少？