—The heavy snow hasn't caused any inconvenience to people in Nanjing, has it?

—Yes. Just imagine the city ________ with nearly 20 centimeters of snow one night.

A．has been hit

B．was hit

C．had been hit

D．is hit

闭环控制系统的时域性能指标是（）

A.相位裕量

B.输入信号频率

C.最大超调量

D.系统带宽