A colleague of Dr. Farah's questions the internal validity of her causal claim. He is curious as to whether the relationship between homework and academic achievement could be explained by interest in one's classes. Specifically, he thinks that students who are interested in their classes will both do more homework and have higher GPAs. Which of the following is a solution to this possible threat to internal validity?
A. Dr. Farah should measure students' interest in their class instead of GPA.
B. Dr. Farah should also measure students' interest in their class.
C. Dr. Farah should study a different sample.
D. Dr. Farah should measure students' interest in their class instead of time spent doing homework.