Man:I thought the librarian said we could check out as many books as we need with our library cards. Woman: That's right, but not those reference books. Question: What does the woman mean

A．Students with a library card can check any book out.

B．Reference books are not allowed to be checked out.

C．Only students with a library card can check out reference books.

D．The number of books a student can check out is unlimited.

下面关于BFD的描述正确的是（）

A.能够对mpls网络进行检测

B.对相邻转发引擎之间的路径提供轻负荷.段持续时间的检测

C.采用单一机制对所有类型的介质.协议层进行检测.实现全网统一的检测机制

D.是一种单向检测机制