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?
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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.