The principal purpose of sub-cooling liquid refrigerant prior to its entering the expansion valve is to （）.
A. increase the refrigerating effect, by decreasing the amount of flash gas
B. allow the refrigerant to enter the throttling device in a saturated condition
C. increase the refrigerating effect by increasing the amount of flash gas
D. minimize the temperature drop of the liquid as it passes through the solenoid valve to the outlet side
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