Five persons are traveling with two luggage bags.Every time,

Five persons are traveling with two luggage bags.Every time,
need two of them to carry the luggage they bags and they do like this in turn during the travel.Now,they have walked 15km.
Question:How many kilometers did each travel with a luggage bag on walk?
Answer:___________
A.15km B.7.5km C.6km D.30km
并说明选择答案的理由