Acandidate key is（).

A.an attribute or a set of attributes with a unique value for each tuple of a relation

B.a set of attributes referring to another relation in the database

C.an attribute or a set of attributes that is potentially the primary key of a relation

D.an attribute or a set of attributes that is potentially(有潜力地)the super key(超键)of a relation

请帮忙给出正确答案和分析，谢谢！