Each machine supporting TCP has a TCP transport entity, either a library procedure, a user process, or part of the kernel. In all case, it manages TCP streams and（）to the IP layer. A TCP（）accepts user data streams from local process, breaks them into pieces not exceeding 64KB, and sends each piece as a separate IP（）. When datagrams containing TCP data arrive at a machine, they are given to the TCP entity, which reconstructs the original byte streams.

The IP layer gives no guarantee that datagrams will be delivered properly, so it is up to TCP to time out and（）them as need be. Datagmms do arrive may well do so in the wrong order, it is also up to TCP to（）them into messages in the proper sequence.

A TCP（）accepts user data streams from local process.

A．connection

B．file

C．entity

D．destination

关于抗感染免疫的叙述，下列不正确的是（）

A.完整的皮肤与黏膜屏障是抗感染的第一道防线

B.抗体与细菌结合可直接杀死病原菌

C.体液免疫主要针对胞外寄生菌的感染

D.细胞免疫主要针对胞内寄生菌的感染

E.抗毒素的中和作用主要针对外毒素感染