An experiment is designed to compare the differences in learning outcomes
between learning math from a video game and learning it from traditional
classroom activities. The experiment finds no difference between the two.

During this experiment, the students in the traditional classroom performed
better in math than they ever have. Which of these may have occurred?

A. A sample size that is too small
B. Treatment group bias
C. The Hawthorne effect
D. Lack of an effective independent variable