Students on Two College Campuses Serve as Treatment and Control

Students on two college campuses serve as treatment and control groups in a study investigating the effectiveness of an alcohol-abuse prevention campaign.A well-known student on one of the campuses dies of alcohol intoxication in the course of the study; students on the other campus did not learn of the student's death.The reaction of other students to the student's death on their campus could represent a potential threat to the internal validity of the study called

A) history.
B) selection.
C) additive effects of selection and history.
D) additive effects of selection and maturation.