（）的作用是防止水倒流，都与闸阀截止阀等串联安装。
A.止回阀
B.节流阀
C.截止阀
D.闸阀
参考答案：A
When you ____________ an agent, usually the appointment is ____________ in writing.
每张行李、包裹票的起码运费是多少？
