I would have come earlier, but I_______ that you were waiting for me. [      ]

A. didn't know    

B. hadn't known    

C. would have known    

D. haven't known

关于功、能关系描述正确的是（）。

A.外力对物体做功时，物体动能减少

B.物体克服外力做功时，物体动能增加

C.除重力和弹力以外的外力对物体做功时，物体机械能减少

D.在只有重力或弹力做功的情况下，物体的动能和势能可以相互转化，而总的机械能保持不变