Norman thinks that the brain function of students suffers when they receive less sleep. He predicts that students who get less sleep before an exam will receive lower test grades. To investigate his idea, he tests two groups of chemistry students. Group 1 receives four hours of sleep, while Group 2 receives the recommended eight hours of sleep the night before the exam. Both groups consume the same foods before testing. They then take the same exam in the same room. What experimental factors could have been a possible source of error in Norman's investigation? A. the amount of sleep students received before the exam. B. the temperature and light of the testing room
C. the grades students earned on their exam
D the food students consumed before the exam