Dr Bach Has a Group of People Listen to Classical

Dr. Bach has a group of people listen to classical music and indicate which of two musicians they like best. After listening to the music, Dr. Bach tells people that they prefer music by Composer A or Composer B. After dividing people up into two groups (in truth, they were divided up randomly) , Dr. Bach finds that people tend to allocate more rewards (concert tickets) to members of their in-group. This in-group favoritism is best explained by:

A) The strong bond that forms when in-group members interact.
B) The mere categorization of people into two groups.
C) Each individual's tendency to reward themselves the most points.
D) A feeling that in-group members share similarities.