Dr. F is interested in whether joining a fraternity/sorority causes people to become more concerned about their attractiveness and appearance. He recruits a group of 55 freshmen (25 males, 30 females)...

Dr. F is interested in whether joining a fraternity/sorority causes people to become more concerned about their attractiveness and appearance. He recruits a group of 55 freshmen (25 males, 30 females) who are planning to go through fraternity/sorority recruitment on his campus. After they join, he gives them a measure of attractiveness concern/appearance concern (the Body Concern Scale).
In addition to measuring the Body Concern of the participants who joined a fraternity/sorority both immediately before and immediately after they join, Dr. F measures them for the 3 weeks before and the 3 weeks after. This type of design would be able to better address which of the following threats to internal validity?

a.  History

b.  Testing

c.  Selection-history

d.  Experimenter bias

B, Testing is the answer but why? I thought C is the answer. (please type)