单项选择题
Refrigerant entering the compressor of a refrigeration system should be in which of the following conditions?（）
A.Sub-cooled liquid B.Sub-cooled vapor C.Superheated vapor D.Liquid
单项选择题 Flame failure in an automatically fired auxiliary boiler can from a/an （）
单项选择题 Cold weather starting of a diesel engine may be made easier by（）
单项选择题 Diesel engine air start system check valves are opened by （）
