Total Keys Possible = $2^5$ = 32
Candidate Key is Minimal Super Key. There can be multiple CK’s.
We select any one key of our choice from CK’s and call it as primary key of the table.
Candidate Keys:
CITY => not possible (As CITY,COUNTRY is a candidate key already)
COUNTRY => not possible(same reason)
Anything added with CITY,COUNTRY is also not possible as we want minimal superkey. Here, we have 8 possibilities.
Possible Candidate Keys = $32 – 2 – 8 = 22$