A user is trying to access a FTP (File Transfer Protocol) server but discovers that FTP (File Transfer Protocol) is being blocked by a firewall. After unblocking port 21 on the firewall, FTP (File Transfer Protocol) still does not work. What is the source of the problem?

A.
TFTP (Trivial File Transfer Protocol) is not installed.

B.
NFS (Network File System) is not installed.

C.
Port 20 must also be unblocked.

D.
Port 22 must also be unblocked.