In which modes does selective packet discard（SPD）operate？（）

A.There is a full mesh of pseudowires.
B.There is a partial mesh of pseudowires.
C.Only L2TPv3 can be used for pseudowire establishment.
D.There is split-horizon to avoid loops.

A.Allow a CSS to probe the server
B.Allow a connection to be closed with a TCP RESET
C.Allow persistent connections
D.Allow the use of HTTP HEAD request