To study the effect of naps on student grades, two experimental treatments are planned: an eight-hour school day with a one-hour nap for one group versus and eight-hour school day with a 30-minute nap for another. Which of the following groups would serve best as a control for this study?

A third group that takes a one-hour nap or a 30-minute nap randomly each day

A third group that shortens the school day to six hours

A third group in which no naps occur

A third group that splits the day with a 30-minute nap in the morning and a 30-minute nap in the afternoon

A third group in which each student chooses either a 30-minute nap or a one-hour nap