Which of the following is the main difference between TCP and UDP?

Which of the following is the main difference between TCP and UDP?

A.
TCP data flows in two directions, while UDP data flows from server to client.

B.
The TCP header implements flags, while the UDP header does not.

C.
The TCP header implements checksum, while the UDP header does not.

D.
TCP connections can be secured by stateful firewalls, while UDP connections cannot.