第3题:[单选题]【32081】定滑车的作用是( )。 ( ) A. 改变绳索或力的方向,且能省力 B.即不能改变绳索或力的方向,也不能省力 C. 不能改变绳索或力的方向,但能省力 D. 改变绳索或力的方向,不能省力 参考答案:D
第4题: [单项选择]Investigators agreed that passengers on the airliner______ at the very moment of the crash. A. should have died B. must be dying C. must have died D. ought to die 参考答案:C 答案解析:must have done用来表示对过去某动作的肯定推测,作“肯定,一定是”讲,例如:He must have been to England for he knows so much about