题目

The patient was warned _______ oily food after the operation.A. to eat not      B. eating not      C. not to eat    D. not eating 答案：C 解析: warn sb not to do sth 警告某人不要做某事。用如图所示滑轮组将重6×102N的物体匀速提高2m，拉力所做的功为2×103J，所用的时间为20s．求：（1）有用功；（2）拉力的功率；（3）该滑轮组的机械效率．