A crow leaves its nest, and flies back and forth from its nest to a nearby ditch to gather worms. The distance between the nest and the ditch is 400 meters. In one and a half hours, the crow manages to bring worms to its nest 15 times. What is the speed of the crow in kilometers per hour?  Choose the correct choice from the following answers:
A) 2 B) 4 C) 6 D) 8 E) 10

Rationale: "the distance between the nest and the ditch is 400 meters . 15 times mean = a crow leaves its nest , and flies back ( going and coming back ) i . e . 2 times we get total 30 rounds . so the distance is 30 * 400 = 12000 . d = st 12000 / 1.5 = t , i think we can take 12000 meters as 12 km , then only we get t = 8 . ( 1000 meters = 1 km ) d )"
Answer: D