Task: Question Answering
Input (English): Problem: 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 ? 
Options: a ) 2 , b ) 4 , c ) 6 , d ) 8 , e ) 10
Output (English): d