2. (15 points) Consider a relation with schema R(A,B,C,D) and FD= { AB->C, C->D, D- >A), what are the keys of R? Hint: Consider the 15 possible non-empty subsets of attributes. First check if a set of attributes X in R is a super key of R. If X-> R then X is superkey of R. Then X is a super key and no subset of X is a super key, then X is a key - a minimum super key