DrFletcher Is Interested in Whether Joining a Fraternity/sorority Causes People

Dr.Fletcher 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 group of participants who joined a fraternity/sorority,Dr.Fletcher decides to give the same measure to another group of 55 participants who decided to not join a fraternity/sorority.After conducting the study,Dr.Fletcher finds out that the people who joined a fraternity/sorority all saw a documentary on body image sponsored by the InterGreek Council the night before recruitment began.This threat to internal validity is known as a:

A) Selection threat
B) Selection-history threat
C) History threat
D) Testing threat