泡沫灭火系统的公称直径DN65～200mm阀门（金属密封）严密性试验的持续时间不应少于（）。
A、30s
B、40s
C、50s
D、60s
参考答案：A
— Must I be in hospital for a week, Doctor?
— No, you . You can go back home tomorrow. [ ]
A. mustn't
B. needn't
C. must
水的预处理主要有哪些方法？
