题目

I was about to leave _____ the telephone rang. A . then B . when C . that D . and when 答案：B1．图是一正弦式交变电流的电压图象．从图象可知电压的最大值和频率分别为（　　）A．40V，100 HzB．20V，100 HzC．40V，50 HzD．20V，50 Hz