Which of the Following Studies Is Best for Showing the First

Which of the following studies is best for showing the first variable produces a change in the second variable?

A) A researcher asks people who are entering the grocery store how hungry they are and then asks them how much they spent on food as they leave the store.
B) A researcher has participants sleep various amounts of hours and then observes how cranky they are.
C) A researcher asks participants how well they like to think and how many books they have read in the last 6 months.
D) A researcher asks participants how many hours per week they listened to Beethoven while growing up and then gives them a math ability test.