Presume that a profit-maximizing monopolist has a plant of the optimal size and is producing a level of output at which price is $30, average fixed cost is $40 and average total cost is $55. The firm must
a. operate in the short run.
b. shut down in the short run.
c. exit the market in the long run.
d. continue to operate in the long run.
e. both a and c.