单项选择题
A.flow of oil
B.pressure of oil
C.A and B
D.temperature of oil
单项选择题 （） are used to limit the pressure in any particular portion and mainly control the pressure after the valve.
单项选择题 Among the basic categories of equipment, which of the following is used to convert hydraulic energy into continuous rotary motion?（）
单项选择题 Among the basic categories of equipment, which of the following are used to convert mechanical into hydraulic energy?（）