题目

—I beg your pardon, but I didn't quite catch you. —Oh, I ________ myself. A . am talking to B . have talked to C . talked to D . was talking to 答案：D5、甲、乙两种机械的效率分别是70%和50%，则下列说法中正确的是（　　）A、使用甲机械省力B、使用甲机械省功C、使用甲机械做功快D、乙机械的额外功在总功中占的比例大