离心水泵按叶轮的数量分有（）。
A、单级泵和多级泵
B、单吸泵和双吸泵
C、卧式泵和立式泵
D、低压泵和高压泵
参考答案：A
It is reported in the newspaper that a man escaped from the prison last night and it was a long time
______ the guards discovered what had happened. [ ]
A. when
B. until
C. since
D. before
皂化值
