For each of the following given relational schemas (R1, R2, R3, R4) with primary key and set of functional dependencies, please indicate what the highest normal form is for each relational schema and why concisely:
a. R1(A, B, C, D)
Key: AB
Functional Dependencies: AB ? C, AB ? D, A ? C, B ? D
b. R2(A, B, C, D)
Key: AB
Functional Dependencies: AB ? C, C ? D
c. R3(A, B, C, D)
Key: AB
Functional Dependencies: AB ? C, AB ? D, C? B
d. R4(A, B, C, D)
Key: AB
Functional Dependencies: AB ? C, AB ? D