IMAT 2017 Q13 [Running Speed]

Luke walks his puppy to a nearby park and lets the puppy off the lead.
As Luke starts walking along the path, the puppy runs on ahead, until it has gone 100 metres. The puppy then turns and runs back to Luke, who in that time has walked 50 metres. The puppy then goes on ahead again, turns after running 100 metres and runs back to Luke who has now walked another 50 metres.

This routine continues until Luke has walked 1 km.

If it takes 12 minutes for Luke to walk 1 km in the park, what is the puppy’s average running speed during the same time?

A. 12 km / h
B. 5 km / h
C. 10 km / h
D. 14 km / h
E. 15 km / h