American Airlines Makes Numerous Nonstop Flights from Chicago's O'Hare Airport

American Airlines makes numerous nonstop flights from Chicago's O'Hare Airport to the airport at Dallas-Fort Worth. The distance between those two cities is 1,000 miles. The only variable cost, fuel, costs $.06 for each passenger-mile it flies. Bob, on his way to an emergency business meeting, buys a ticket in coach class for $1,300 at the very last minute. The marginal cost of flying Bob from Chicago to Dallas-Fort Worth is:

A) $60.
B) $160.
C) $600.
D) $1,300.