At noon, ship A is 100km west of ship B. Ship A is sailing south at 40km/h and ship B is sailing north at 20km/h. How fast (in km/h) is the distance between the ships changing at 4:00pm? Round your answer.
a. 30 km/h
b. 40 km/h
c. 50 km/h
d. 60 km/h