单项选择题
“A centrifuge consists of an electric motor driving a vertical shaft on the top of which is mounted the bowl assembly”.According to this sentence, we can know that is mounted on the top of the vertical shaft（）
A.a centrifuge B.an electric motor C.a bowl assembly D.a driving motor
单项选择题 The treatment of the fuel oils involves storage, heating, filtering and（）
单项选择题 When anaerobic bacteria convert the sewage to an inert sludge,（）
单项选择题 Bacteria which live are said to be aerobic（）
