An instrument that maintains a continuous record of humidity changes is called a（）.

A.weather vane，wind vane or wind sock
B.hydrometer
C.hygrometer
D.sling psychrometer

A.set the relief valve
B.check the lubrication system
C.vent the pump casing
D.open the pump suction and discharge valves

A.increase engine speed to insure adequate flow of oil to all parts of the engine
B.pay no attention unless there are unusual noises from the engine
C.check operating pressures and temperatures，and check for leaks
D.run the engine at idle until the temperature has increased