Wendy Wants to Study How Easily People Make Friends

Multiple Choice

Wendy wants to study how easily people make friends. She assigns people to talk with a stranger for either five minutes or ten minutes, and assigns people who are more outgoing to the "five-minute" condition, and shy people to the "ten-minute" condition. Which of the following is the threat to the internal validity in this study?

A) The independent variable isn't held constant for every participant in a condition.
B) Dependent variables are not being assigned randomly.
C) She did not select from a random sample in her population.
D) Wendy did not use random assignment.