单项选择题
A.cause a loss of the liquid seal
B.create a vapor lock in the liquid receiver
C.collect in the condenser
D.cause foaming of the oil in the crankcase
单项选择题 Pressure in an operating hydraulic system is developed （）.
单项选择题 If a large difference exists between the compressor discharge pressure and the pressure corresponding to the existing condensing temperature in a refrigeration system, you should（）.
单项选择题 Which of the following statements concerning butterfly valves is correct?（）