在shell编程时，使用方括号表示测试条件的规则是：方括号两边必须有（）。
参考答案：空格
— How clean and tidy your classroom is!
— Thank you. It ______ every day. [ ]
A. is cleaned
B. was cleaned
C. was cleaning
D. is cleaning
QC83系列防爆开关中Q的含义为（），C的含义为磁力，8的含义为隔爆代号。
