In a Study of the Effect of Fatigue on Task

In a study of the effect of fatigue on task performance, participants were asked to complete a series of puzzles. One day, all participants completed puzzles after 24 hours without sleep. On another day, the same participants completed puzzles after sleeping for at least eight hours. What can be said about the research methods used in this study?

A) The control and experimental groups were entirely confounded with one another.
B) There was no control group, so the study is seriously flawed.
C) Participants served as their own control group, in order to examine the effect of the independent variable under different conditions for the same individuals.
D) Participants provided their own matched controls, so that the researchers were unable to determine whether the independent variable had any functional impact.