Let R = (A, B) and S = (A, C), and let r(R) and s(S) be relations. Write relational-algebra expressions equivalent to the following domain-relationalcalculus expressions:

a. {< a > | ∃ b (< a, b >∈ r ∧ b = 17)}

b. {< a, b, c > | < a, b >∈ r ∧ < a, c >∈ s}

c. {< a > | ∃ b (< a, b >∈ r) ∨ ∀c (∃ d (< d, c >∈ s) ⇒ < a, c >∈ s)}

d. {< a > | ∃ c (< a, c > ∈ s ∧ ∃ b1, b2 (< a, b1 > ∈ r ∧ < c, b2 > ∈ r ∧ b1 > b2))}