When they visited their hometown, they saw many changes ______ in that area. [ ]

A. were happened

B. which was happened

C. which had been happened

D. which had happened

为了提高接收机的灵敏度，尤其是在外来信号很微弱时，增益控制电路应采用（）。

A..普通自动增益控制电路

B..提高放大器电源电压

C..减小放大器衰减

D..延迟式自动增益控制电路