安装断路器离地的最小距离为（）m。
A.1.5
B.2.0
C.2.8
D.3.80
参考答案：C
— Tom is never late for work. Why is he absent today ?
— Something _______ to him.
A．must happen
B．should have happened
C．could have happened
D．must have happened
仪表的精度等级与仪表（）的大小有关。
A.允许误差
B.引用误差
C.基本误差
D.绝对误差
