If A and B are candidate keys, then AB is candidate key or not ? If R{abc} is a relation, f{ a->b b->a a->c b->c }, then a and b are candidate keys. What about ab ? Is it a candidate key or is it a super key or both ?
The union between A and be is a superkey.

(A candidate key is a superkey but a superkey could be not a candidate key)

Moreover, remember that a candidate key is a particular type of superkey, so you shouldn't worry about if it is "both" key and superkey.