Mary wants to find out whether eating sugary snacks before an exam leads to better performance on the exam. Which of the following strategies would answer her question most conclusively? a. Identify a...

Mary wants to find out whether eating sugary snacks before an exam leads to better performance on the exam. Which of the following strategies would answer her question most conclusively?
a. Identify a large number of students who perform exceptionally low and exceptionally high in exams,ask them whether they eat sugary snacks before exams, and see whether high performers eat more sugary snacks before exams than do low performers.
b. Wait for exam time in a big class, ask everyone whether they ate sugary snacks before the exam, and see whether those who ate sugary snacks before the exam do better compared to those who didn’t.
c.Wait for exam time in a big class, give a random half of the students M&Ms before the exam, and see whether the students who ate M&Ms perform better.
d. Pick a big class, give all students sugary snacks before one exam and salty snacks before the next exam; then see whether students score lower on average in the second exam.