用活塞式压力计检测井口压力，加减砝码的原则是（）。
A、先减后加
B、用手按住托盘减砝码
C、先加后减
D、加砝码时无需按住托盘
参考答案：B
The workers were making so much noise in the workshop, and the boss hurriedly
Went to see____.
A．what thematter was
B．what was the wrong
C．what wrong was
D．what was the matter
CW7912稳压管表示输出（）电压值为12V。
A.固定正电压
B.固定负电压
C.固定负电流
D.固定正电流
