The complete SIP message interaction process will include multiple request and response messages. A field can be used in the header field of the SIP message to describe the specific request corresponding to the current response message. This field is:

The complete SIP message interaction process will include multiple request and response messages. A field can be used in the header field of the SIP message to describe the specific request corresponding to the current response message. This field is:
A . Csea
B . CaLL-ID
C . First line
D . Message body

Answer: A