单项选择题
When two oil separators are working in series, they should work with （）
A.both as clarifiers B.first one as clarifiers, second one as purifier C.both as purifiers D.first one as purifier, second one as clarifier
单项选择题 Concerning the management of sewage treatment plant using activated sludge, which of the following is correct?（）
单项选择题 Disinfector in the sewage biological treatment plant usually is （）
单项选择题 Before marine sewage treatment plant works （） should be filled with fresh water.
