为减少辐射散热，在高温炉门前设置隔热挡板是简单有效的办法。挡板材料的黑度越低，则炉门的散热量将（）。

A.越少

B.越多

C.变化不大

D.无法判断

Frame Relay is simplified form of (71) , similar in principle to (72) , in which synchronous, frames of data are routed to different destinations depending on header information . Packets are routed throught one or more Virtual Circuits known as (73) . Most Virtual Circuits are (74) ,which means that the network provider sets up all DLCI connections at subscription time. (75) are also part of the Frame Relay specification . They privide a link that only lasts only as long as the session.

A．X.21

B．X.25

C．X.28

D．X.29