about this computer program, a huge amount of time and energy　　　　.

A．Should I know; be saved

B．If I had known; can be saved

C．Had I known; would have been saved

D．Did I know; was saved

要保证抽样调查的结果能真实反映总体情况，需要（）

A.样本具有代表性

B.样本量足够大

C.调查和测量的方法具有较好的可靠性

D.抽样要遵循随机化原则

E.以上均正确