单项选择题 Before the sludge in a purifier is ejected,（）

单项选择题 A centrifuge of which the bowl is arranged as the upper and lower parts separate discharges the sludge（）

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（）