After visiting China, most of the foreign friends said they would never forget the time      _________they had spent in China.

A．what

B．that

C．when

D．at which

质量为m的物体，在距地面h高处以

A．物体的重力势能减少mgh

B．物体的动能增加

C．物体的机械能减少

D．重力做功