Rick’s class plans to study the effect of eating different breakfast foods on grades in morning classes. Students in the class will be the test subjects for the investigation. Which of the following would most likely make the results of the experiment invalid?

The investigation is testing how breakfast foods affect grades, so students should eat breakfast at roughly the same time before class. If some eat two hours earlier than others, the time between eating and the morning class could also affect their grades, making it harder to tell whether the food caused any difference. Therefore, the choice about students eating breakfast at different times is most likely to make the results invalid.

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