单项选择题
A.one year
B.two years
C.four years
D.five years
单项选择题 （）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.
单项选择题 For a given exhaust gas boiler, what is the quantity of its evaporation mainly determined?（）