如下图所示,在Cisco2511路由器R1的16个异步串行接口上连接一台Modempool为用户提供拨号上网服务,分配的IP子网地址为195.223.56.16/28;使用同步串行接口,通过DDN专线与R2相连,分配的IP地址为222.112.41.81/30。下列路由器R1的异步接口a1和同步接口s1/1的部分配置,正确的是()

A.Router(config)#interface a1
Router(config-if)#encapsulation ppp
Router(config-if)#async default ip address 195.223.56.16
Router(config-if)#exit
Router(config)#interface s1/1
Router(config-if)#ip address 222.112.41.81 255.255.255.252
Router(config-if)#encapsulation ppp
B.Router(config)#interface a1
Router(config-if)#encapsulation ppp
Router(config-if)#async default ip address 195.223.56.16
Router(config-if)#exit
Router(config)#interface s1/1
Router(config-if)#ip address 222.112.41.81 255.255.255.252
Router(config-if)#encapsulation hdlc
C.Router (config)#interface a1
Router(config-if)#encapsulation ppp
Router(config-if)#asyuc default ip address195.223.56.17
Router(config-if)#exit
Router(config)#interfacd s1/1
Router(config-if)#ip address 222.112.41.81 255.255.255.252
Router(config-if)#encapsulation ppp
D.Router (config)#interface a1
Router(config-if)#encapsulation hdlc
Router(config-if)#async default ip address 195.223.56.17
Router(config-if)#exit
Router(config)#interface s1/1
Router(config-if)#ip address 222.112.41.81 255.255.255.252
Router(config-if)#encapsulaton ppp