Let R = (A, B, C), and let r1 and r2 both be relations on schema R. Give an expression in the domain relational calculus that is equivalent to each of the following:

a. $\prod _A(r1)$

b. $\sigma _{B =17} (r1)$

c. r1 ∪ r2

d. r1 ∩ r2

e. r1 − r2

f. $\prod _{A,B}(r1) \Join \prod _{B,C}(r2)$