My mother ______ have dinner at home. [ ]
A. isn't
B. don't
C. doesn't
D. aren't
答案:C
下套管时要按照套管表下入顺序下套管,注意按指定的位置加装(),加装时要()。
电器设备的防爆总标志是()。
A.MX
B.EX
C.I
D.d