My father served in the army in        when he was in         .

A．1950s; twenties

B．the 1950s; his twenties

C．the 1950s; the twenties

D．1950s; the twenties

NPN型晶体管处于放大工作状态时，总是( )。

A．发射极加正向电压，集电结加反向电压

B．发射极加反向电压，集电结加正向电压

C．发射极加正向电压，集电结加正向电压

D．发射极加反向电压，集电结加反向电压