Economists say competitive markets are efficient because:

a. by producing in the output range where MB > MC, profits are earned and the difference between consumer surplus and producer surplus is maximized.

b. by producing in the output range where MB < MC, profits are reduced and the difference between consumer surplus and producer surplus is maximized.

c. by producing up to the point where MB > MC, profits are earned and the difference between the consumer surplus and producer surplus is maximized.

d. by producing up to the point where MB = MC, profits are maximized and the difference between the consumer surplus and producer surplus is maximized.