I saw him leave the office an hour ago . He ____ at this time .

A．mustn''t be working

B．can''t be working

C．can''t work

D．may not being working

TCP (Transmission Control Protocol) was specifically designed to provide a reliable end-to-end byte stream over a(n) （）.

A.IP address

B.reliable network

C.socket

D.unreliable internetwork