Buster Runs an Experiment in Which She Finds That Participants

Multiple Choice

Buster runs an experiment in which she finds that participants placed in a good mood by a happy film clip are more likely to offer help to a fellow participant than participants who see a sad film clip. Bob suggests that perhaps this difference simply results from the fact that the participants in the happy film clip condition were more helpful people to begin with. What methodological aspect of a successful experiment can Buster point to in order to refute this criticism?

A) External validity
B) Random selection
C) Random assignment
D) Manipulation