Sam _____ some knowledge of the computer just by watching others working on it. [ ]

A. brought up

B. looked up

C. picked up

D. set up

雷达的脉冲调制器在（）的作用下，控制各级功率放大器，将频率源送来的射频信号放大，输出大功率的射频脉冲信号。

A.功率信号

B.射频信号

C.视频信号

D.触发脉冲