(a) For a perfectly competitive market, a rotation in the demand curve from D₁ to D₂ does not change the equilibrium quantity and price .(b) For a firm with market power, a rotation in the demand curve from D₁ to D₂ rotates the marginal revenue curve from MR₁ to MR₂. Prior to the rotation, the profit-maximizing quantity and price occurred where MR₁ = MC. After the rotation, the firm is profit-maximizing at a higher quantity and lower price where MR₂ = MC.