Which of the Following Studies Is Best for Showing That

Multiple Choice

Which of the following studies is best for showing that the first variable causes changes 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 assigns participants to 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.