The Border Gateway Protocol (BGP) is an interautonomous system. ___49___ protocol. The primary function of a BGP speaking system is to exchange network ___50___ information with other BGP system. This network reachability information includes information on the list of Autonomous System (ASs) that teachability information traverses. BGP-4 provides a new set of mechanisms for supporting ___51___ interdomain routing. These mechanisms include support for advertising an IP ___52___ and eliminate the concept of network class within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including ___53___ of AS paths. These changes provide support for the proposed supernettting scheme.

50（）

A. teachability

B. scalablity

C. availability

D. reliability

当降冰粒时，若其内部有未冻结的水在地面碰破后则应同时记录冰粒和雨。