Actually, there is no ambiguity in the question.
What was the study? 60 students were put on nutritious diet. 60 were not.
What was the result? The 60 students who were put on nutritious diet did not have higher grades than other 60.
What did they conclude? Special nutritional planning does not positively affect students’ grades.
Now, we need to evaluate the accuracy of the study. What will help us figure out whether what we concluded from the study is valid or not?
Because 60 students with diet plans did not get better grades, can we say nutrition does not have any positive affect on grades? Perhaps not. What if nutrition can improve low grades (by giving students more energy and making them more active) and bring them up to average (but not improve average to take them to above average since they need to work hard too)? If this were true, the conclusion of the study would not be valid. Special nutritional planning WOULD have a positive affect on students’ grades. "If grades were low, they could become better by nutritional planning"
Hence option (D) is useful in determining the accuracy of the study.
As for other options, focus on what was concluded from the study. Are they relevant to the conclusion "Special nutritional planning does not positively affect students’ grades."?
(A) Performance of business executives was shown to improve drastically after major alterations were made in their diets.
Not relevant to students' grades.
(B) Honors students, after altering their diets, maintained that they did not change their study habits.
Students saying that they did not alter their diet doesn't mean anything. It is true that they said it but did they actually maintain their study habit or not, we don't know. So not relevant. No need to think further.
(C) Students who participated in various fitness regimens found that their grades improved appreciably after they altered their exercise habits.
Nothing to do with fitness regime and grades. Only food and grades connection is our concern.
(E) All of the college students who volunteered for the study were either in their first or second year of college.
Again, not relevant.
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