He traveled to many mountain villages and saw many poor children out of school. This experience _____ his life. [ ]

A. would change

B. had changed

C. was to change

D. was changing

你对天线的增益dBd是如何认识的（）。

A．这是一个绝对增益值

B．天线是无源器件，不可能有增益所以此值没有实际意义

C．这是一个与集中幅射有关的相对值