You want to know if listening to music makes you solve puzzles quickly. What is the theoretically correct counterfactual comparison to make?
A. Compare the music listening habits of people who solve puzzles quickly with those who do not
B. Compare the puzzle solving ability of those who listen to music with those who do not.
C. Compare the listening habits of the same people while they solve and do not solve puzzles
D. Compare the puzzle solving ability of the same people while they listen and do not listen to music
Answer: D. Compare the puzzle solving ability of the same people while they listen and do not listen to music