—Why is the classroom so dirty?

—Sorry，sir. It       yesterday, We forgot to do it.

A．doesn’t clean

B．didn’t clean

C．isn’t cleaned

D．wasn’t cleaned

要稳定输出电流，减小电路输入电阻应选用（）负反馈。

A、电压并联

B、电流串联

C、电流并联

D、电压串联