30 POINTS To study the effect of cell phone use on the time it takes to fall asleep, two experimental treatments are planned: one-hour cell phone use before bedtime for one group versus two-hour cell phone use for another. Which of the following groups would serve best as a control for this study?

A third group that uses a cell phone randomly for one hour or two hours before bedtime
A third group that uses a cell phone for 30 minutes before bedtime
A third group that uses a cell phone only if awoken in the middle of the night
A third group in which each subject chooses to use a cell phone for one hour or two hours before bedtime
A third group that does not use a cell phone before bedtime