单项选择题
A.no oxygen is needed
B.oxygen is necessary
C.oxygen is given off
D.no gas is produced
单项选择题 Bacteria which live are said to be aerobic（）
单项选择题 It is that reduce the BOD by converting the organic content of the sewage to a chemically and organically inert sludge（）
单项选择题 The basic process of biological sewage treatment consists of the liquor either by bubbling air through it or by agitating the surface（）