电动式振动探测器的安装,室外应用时通常埋入地下,但不宜埋入土质松软地带,深度在( )左右。
A.50mmB.85mmC.100mmD.120mm
参考答案:C
It ______ be the postman at the door. It's only six o'clock.[ ]
A. mustn't
B. can't
C. won't
D. needn't
推断统计