单项选择题
A knocking sound emitted from an unloaded air compressor is probably caused by （）.
A.damaged intake valves B.excessively tight mounting bolts C.insufficient cylinder lubrication D.a loose piston
单项选择题 The refrigeration system low pressure cutout switch is actuated by （）.
单项选择题 Hydraulic machinery failures are commonly caused by contamination of the hydraulic fluid and （）.
单项选择题 When you adjust a thermostatic expansion valve, （）.
