Which of the following query transformations, obtained by replacing the…

Which of the following query transformations, obtained by replacing the left-hand side expression by the right-hand side expression, is incorrect?

R₁ and R₂ are relations. C₁ and C₂ are selection conditions, and A₁ is an attribute set of R₁.

a. σ_{C₁}(σ_{C₂}(R₁)) → σ_{C₂}(σ_{C₁}(R₁))

b. σ_{C₁}(π_{A₁}(R₁)) → π_{A₁}(σ_{C₁}(R₁))

c. σ_{C₁}(R₁ ∪ R₂) → σ_{C₁}(R₁) ∪ σ_{C₁}(R₂)

d. π_{A₁}(σ_{C₁}(R₁)) → σ_{C₁}(π_{A₁}(R₁))