4. [20 points] Consider a relation schema R (A, B, C, D). For each of the following four sets of functional dependencies, identify the best normal form that R satisfies (1NF, 2NF, 3NF, or BCNF), and point out the normal form violations. (a) CD B, B A, AC (b) AB CD, C A (c) B-CD, CA, AB (d) ABC, ABD, BC=AB