An airplane maintains a horizontal speed of 500 km/h due north at a constant altitude of 4 km. At the instant the plane passes directly over an airport, a car begins driving due east from the same airport at 80 km/h. How fast is the straight-line distance between the airplane and the car changing 10 minutes after they start?
a) 500 km/h
b) 583 km/h
c) 505 km/h
d) 493 km/h