（）connectors hold best if a wire is accidentally pulled or a connection becomes loose.

单项选择题（）are used to digest the sewage to produce innocuous sludge in a biological sewage treatment plant.

A.anaerobic bacteria
B.aerobic bacteria
C.air bubbles
D.disinfection chemicals

单项选择题For a given exhaust gas boiler, what is the quantity of its evaporation mainly determined?（）

A.the temperature and flow quantity of the exhaust gas from M.E
B.the quantity of steam demanded
C.the rotary speed and the cooling water temperature of main engine
D.efficiency of M.E turbocharger

单项选择题A purge recovery system is used in a centrifugal air conditioning or refrigeration system to （）

A.purge lube oil from the liquid refrigerant
B.recover water purged from the system
C.separate foul gases from the receiver
D.purge non-condensable gases without losing refrigerant

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 （）

A.incorrect electrode setting
B.incorrect nozzle position
C.clogged fuel nozzle
D.broken high tension lead