-May we leave the classroom now?

-No, you ______. You ______ to leave until the bell rings.

A．mustn’t, are allowed

B．don’t have to, are supposed

C．needn’t, aren’t allowed

D．can’t, aren’t supposed

并联的目的是为了提高水泵的扬程，串联的目的是为了增加水泵的流量。