American Airline flies between New York and Toronto. The company leases planes on a year-long contract at a cost that averages $600 per flight. Other costs (fuel, flight attendants, etc.) amount to $550 per flight. Currently, American Airline’s revenues are $1,000 per flight. All prices and costs are expected to continue at their present levels. If it wants to maximize profit, American Airline should ( )
选项：

A:shut down immediately.;
B: continue the flight.;
C:continue flying until the lease expires and then exit.;
D:shut down now but renew the lease if conditions improve.