A track team captain thinks that eating breakfast and having a good night's sleep may affect the team's performance. The track team decides to test if eating breakfast makes a difference in how fast they can run one mile.

They decide to split the team into two groups. One group eats breakfast two hours before running and the other group does not eat breakfast. The students also make a rule that both groups have to get a good night's sleep before they run. Is this rule necessary?

A. Yes, because the rule allows the team to only test one variable at a time, keeping everything else a constant.
B. No, because they can judge the effect of having breakfast even if the students do not get enough sleep.
C. No, because they can judge the effect of having breakfast only if some students do not get enough sleep.
D. Yes, because they can judge the effect of both breakfast and sleep if all students get enough sleep.