Two trains leave a city at 1 p.m. One train goes north at a constant rate of 75 mph, and the other goes south at a constant rate of 63 mph. At what time will the trains be 621 miles apart?

a) 4:30 p.m.
b) 5:30 p.m.
c) 6:30 p.m.
d) 7:30 p.m.