妊娠时维持黄体功能的主要激素是

A．雌激素

B．孕激素

C．FSH

D．LH

E．绒毛膜促性腺激素

A transport layer protocol has several responsibilities, One is to create a process-to-process (program-to-program) communication; TCP uses port （1） to accomplish this. Anotherresponsibility of a transport layer protocol is to create a（2）and error-control mechanism atthe transport level. TCP uses a sliding（3）protocol to achieve flow control. It uses theacknowledgment packet, time-out, and retransmission to achieve （4） control. The transportlayer is also responsible for providing a connection mechanism for the application program. Theapplication program sends （5）of data to the transport layer. It is the responsibility of thetransport layer at the sending station to make a connection with the receiver.

（4）是（）

A.packet

B.time

C.error

D.phase