题目

How long ______at this factory? A . have you come B . has he in C . will he be D . had he been 答案：C正弦交流电是由闭合线圈在匀强磁场中匀速转动产生的。线圈中感应电动势随时间变化的规律如图所示，可知该交变电流A．周期是0．01sB．电压的最大值是311VC．电压的有效值是220VD．电流的瞬时值表达式