A user has a disk quota of 15 MB. They are already using 12 MB and decide to compress their files to make room for more. After compressing the files, the user realizes one of their documents is an important sales proposal. He decides to encrypt the file without success. What is the most likely cause?

a. Compression has no effect on disk quota.

b. Only a system administrator can encrypt files.

c. You can't encrypt a compressed file.

d. You have to encrypt the file before compressing it.