Which of the following is true?

a. Economic profits encourage the entry of new firms, which shift the market supply curve to the right.

b. Any positive economic profits signal resources into the industry, driving down prices and revenues to the firm.

c. Any economic losses signal resources to leave the industry, leading to supply reduction, higher prices, and increased revenues.

d. Only at zero economic profits is there no tendency for firms to either enter or exit the industry.

e. All of the above are true.