When profits are being earned in a monopolistically competitive industry,
a. It is impossible to earn profits in monopolistic competition.
b. Profits increase because new firms entering the industry cause the demand for the product(s) in the industry to increase, thus driving up prices.
c. The profits persist because significant barriers exist that prevent the entry of new rivals into the industry.
d. Profits erode until economic profits are equal to zero because there are few if any barriers to entry preventing new firms from entering the industry.