单项选择题
Black smoke coming from a boiler can be caused by （）.
A.low water level B.low oil pressure C.excessively high fuel pressure D.an improper air/fuel ratio
单项选择题 Which will cause the high pressure cutout to stop the compressor in a R-22 system?（）
单项选择题 Insufficient refrigerant in a refrigeration plant may result in （）.
单项选择题 Air that may enter a refrigeration plant tends to collect in the （）.
