题目

如图所示的交变电流由正弦交变电的一半和反向脉冲电组合而成，则这种交变电流的有效值为（ ） A . B . C . D . 答案：CDavid said that it was because of his strong interest in literature＿＿he chose the course.A．thatB．whatC．whyD．how