单项选择题
A.operated manually once an hour
B.operated until the engine has started
C.placed into operation only at maximum load
D.adjustable to meet lubrication requirements
单项选择题 Bottom blow valves are installed on auxiliary water-tube boilers to （）
单项选择题 A three-way thermostatic control valve regulates the diesel engine cooling water temperature by passing a portion of the water （）.
单项选择题 The concentration of total dissolved solids in the water of an auxiliary boiler can increase as a result of （）.