Suppose that a $4 per unit tax is imposed on the sellers of DVDs. The effect of the tax will be to a. ​shift the supply curve up by exactly $4 and the price paid by buyers will remain unchanged. b. ​shift the supply curve up by exactly $4 and the price received by sellers will rise by exactly $4. c. ​shift the supply curve up by exactly $4 and the price paid by buyers will rise by less than $4. d. ​shift the demand curve down by exactly $4 and the price paid by buyers will fall by exactly $4.