Two boats depart from the same pier at noon. Boat A travels due east at a constant speed of 20 km/h, and Boat B travels due north at a constant speed of 15 km/h. How fast is the straight-line distance between the two boats changing at 2:00 p.m.?
a) 17.9 km/h
b) 25 km/h
c) 35 km/h
d) 5 km/h