15. —When Lily fainted, we could have turned to your father for help.

—Yes, sending her to hospital _________ necessary.

A．hadn’t been

B．wasn’t

C．wouldn’t be

D．weren’t

在交流放大器中引入负反馈，可以（）。

A.稳定静态工作点

B.放大倍数

C.输出电压

D.稳定静态工作点、放大倍数、输出电压和输出电流