When a BGP router is not capable of understanding 4-byte AS numbers， it will see 4-byte AS numbers as aspecial， reserved， 2-byte AS number in the AS path. Which 2-byte AS number is this reserved one？（）

A. The Layer 2 multicast address of the destination node
B. The solicited node multicast address of the destination node
C. The Layer 2 address of the destination node based on the destination IPv6 address
D. The IPv6 address of the destination node based on the destination Layer 2 address

A.6to4
B.6over4
C.ISATAP
D.GRE