止回阀的作用？
参考答案：保证流体沿着一定方向流动，防止流体倒流，保护设备。
________ I really don't like art, I find his work impressive.
A．As
B．Since
C．If
D．While
皮肤病的他觉症状是（）
A.痛
B.烧灼
C.麻木
D.糜烂
E.蚁走感
