The diagram at right shows the structure of cost and demand facing a monopolistically competitive firm in the short run.

The profit$-$maximizing output level is $20$ units of output. $($Enter your response as an integer.$)$

The profit$-$maximizing price is $$12.($Enter your response as an integer.$)$

Total revenue is $$240.($Enter your response as an integer.$)$

Total cost is $$280.($Enter your response as an integer.$)$

Total profit or loss is $$40.($Enter your response as an integer and include a negattive sign where appropriate.$)$ In the long run, firms will

A$.$ enter, shifting the demand facing the remaining firms to the left until the firms earn an economic profit.

B$.$ exit, shifting the demand facing the remaining firms to the right until the firms earn an economic profit.

C$.$ exit, shifting the demand facing the remaining firms to the right until the firms earn a normal profit.

D$.$ enter, shifting the demand facing the remaining firms to the left until the firms earn a normal profit.