Martin purchased three units of good A. The marginal benefit of the fourth unit of A exceeds the marginal cost Of the fourth unit Of good A. Which of the following reasons explains why Martin should purchase the fourth unit? l. The marginal net benefit Of the fourth unit is positive. II. Buying the fourth unit will increase total benefits by more than total costs. Ill. Buying the fourth unit will increase total benefits and decrease total costs. A. I Only B. I and II only C. II only D. l, II, Ill