—Cathy, did you attend the lecture given by Mr Black last night? 

—No. I ______ a report at the office.

A．was writing

B．wrote

C．had written

D．have been writing

一个按正弦规律变化的交流电的图象如图所示，根据图象可以知道（   ）

A．该交流电流的频率是0.02Hz

B．该交流电流的有效值是14.14A

C．该交流电流的瞬时值表示式是i="20sin0.02t" (A)

D．该交流电流的周期是0.01s