谐振功率放大器输入激励为余弦波，放大器工作在临界状态时，集电极电流为（）.
A.余弦波
B.尖顶余弦脉冲波
C.有凹陷余弦脉冲波
D.三角波
参考答案：B
Only when the war was over ____ to his home town.
A．did the young soldier return
B．the young soldier returned
C．returned the young soldier
D．the young soldier did return
这书内容好，______又不贵，你买一本吧。
A．况且
B．以至
C．以致
D．不如
