停气动火作业采用直接置换法时,需经连续()次取样检测混合气体中燃气的浓度(每次间隔五分钟),测定结果均符合要求才可动火。
A、二
B、三
C、四
D、五
参考答案:B
— Is your friend a boy?
— _____.[ ]
A. Yes, he isn't
B. No, she isn't
C. A boy
D. Yes, she is
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