A. An IPv4-compatible tunnel is a static tunnel， but an 6to4 tunnel is a semiautomatic tunnel.
B. The deployment of a IPv4-compatible tunnel requires a special code on the edge routers， but a 6to4 tunnel does not require any special code.
C. An IPv4-compatible tunnel is typically used only between two IPv6 domains， but a 6to4 tunnel is used to connect to connect two or more IPv6 domains.
D. For an IPv4-compatible tunnel， the ISP assigns only IPv4 addresses for each domain， but for a 6to4 tunnel， the ISP assigns only IPv6 addresses for each domain.