​ Refer to Exhibit 5-5. Suppose that D2 and S1 are the prevailing demand and supply curves for a product. If the demand schedule changes from D2 to D1, then: a. ​ equilibrium quantity increases from 13 to 18. b. ​ equilibrium price decreases from $6 to $4. c. ​ equilibrium quantity decreases from 15 to 13. d. ​ equilibrium price increases from $6 to $8.