Which is a correct statement about sliding windows used with TCP?

A. The window size is established during the three-way handshake.
B. Sliding windows allow for data of different lengths to be padded.
C. It allows TCP to indicate which upper-layer protocol created therequest.
D. It allows the router to see the segment as urgent data.

A. The window size, which is a buffer, is established and agreed upon by the sender and receiver during the three-way handshake.