If a profit-maximizing, competitive firm is producing a quantity at which marginal cost is between average variable cost and average total cost, it will 

A． a. keep producing in the short run but exit the market in the long run.
B． b. shut down in the short run but return to  production in the long run.
C． c. shut down in the short run and exit the market in  the long run.
D． d. keep producing both in the short run and in the  long run.