密封的概念？
参考答案：
系指防止设备及管道内部介质的泄漏或外界杂质的浸入所采用的装置。一般分为动密封和静密封。
--- The two old friends were busy with each other the time.
--- Yes. They haven’t met for long.
A．too, to talk; to forget
B．too, talking; to forget
C．so, to talk; that they forgot
D．so, talking; that they forgot
尼龙管适用于中低压，一般可达（）MPA。
A、15
B、2
C、8
D、10
