Nowadays many parents from the countryside cannot afford to educate their children, because the school      are too high.

A．fees

B．prices

C．charges

D．fares

重力供液的特点是（）。

A.蒸发器内闪发气体小

B.压缩机不易发生湿冲程

C.气液分离器必须高于蒸发器

D.只适用于双级压缩