题目

Two men stood under a tree smoking and chatting for more than an hour, and then         A. left               B. leaving            C. to leave          D. leave 答案：A小红用模型电动机做了一个小电风扇，通电后她发现风扇向后吹风，下面哪个方法可以使风扇向前吹风（ ）A．增大通过线圈的电流B．换用电压更高的直流电源C．增强磁场D．将电源两极对调一下