At noon a boat is 10 km east of a pier and sailing due east at 30 km/h. At the same time a second boat is leaving the pier and sailing due north at 40 km/h. How fast is the distance between the two boats changing at 1:00 p.m.?
a) 70 km/h
b) 56.6 km/h
c) ≈49.5 km/h
d) 50.0 km/h