Which of the following commands can be used for that purpose?

Assume that you have a file called zips.txt that contains several postal zip codes, and you need to determine how many unique zip codes there are in the file.

Which of the following commands can be used for that purpose?
A . uniq zips.txt
B . count zips.txt
C . sort zips.txt | uniq -c
D . cat zips.txt | uniq -c

Answer: C