— Must they clean and tidy the classroom now?

— No, they      . It       after school in the afternoon.

A．mustn’t; must clean

B．needn’t; must clean

C．mustn’t; need be cleaned

D．don’t have to; can be cleaned

全波整流器的一个缺点是要求变压器次级必须有两个对称的绕组，使造价增高。