$12.$ At a point where a competitive market is in long run equilibrium, which of the following is correct?

a$.$ a firm's demand curve is tangent to the downward portion of its ATC curve

b$.$ a firm's demand curve is tangent to the lowest point of its ATC curve

c$.$ a firm's demand curve is tangent to the lowest point of its MC curve

d$.$ a firm's earns zero economic profits, even though its accounting profits could be positive or negative

e$.$ firms in the industry have no incentive to enter or exit the market