塔顶放大器从技术原理上是降低基站接收系统噪声系数，从而提高基站接收系统灵敏度。
参考答案：对
His sister wanted to have him ____ the radio, but he had had it ____ already.[ ]
A. repair, repair
B. to repair, repair
C. repaired, to repair
D. repair, repaired
图7是某种正弦式交变电压的波形图，由图可确定该电压的
A．周期是0.01S
B．最大值是311V
C．有效值是220V
D．表达式为U=220sin100πt（V）
