单向阀的作用是使油液只能向一个方向流动，而不允许倒流，也常当背压阀使用。
参考答案：对
When help，one often says" Thank you,"or" It's kind of you." [ ]
A. offering
B. to offer
C. to be offered
D. offered
“医经者，原人血脉、经落”中“原”意思是（）选项：
A.原来
B.推原
C.原因
D.本原
E.缘故
