空压机通常在（）后面设安全阀。
A．第一级
B．最末级
C．每一级
D．A或B或C
参考答案：C
----- I must go now, thanks again for your hospitality.
------ _________.
A．My pleasure! So long!
B．With pleasure! Goodbye!
C．You are welcome! Go slowly!
D．Have a nice day! Good luck!
