The accompanying graph shows the cost curves for Mei's mushroom gathering business, which is perfectly competitive.

The graph plots quantity in bushels per month versus price in dollars per bushel. Three curves marked A$,$ B and C are shown in the graph.

If mushrooms sell for $$40$ per bushel, and Mei chooses the profit$-$maximizing quantity, she will

Select one:

a$.$ earn negative profit $($a loss$).$

b$.$ shut down.

c$.$ earn zero profit.

d$.$ earn positive profit.