If, in the Example of a Headache Study Used in the Text

If, in the example of a headache study used in the text, all subjects had been studied over the same period of time, differences that might be caused by stressful times (such as Christmas) would

A) contaminate the results.
B) add to the usefulness of the results.
C) improve the generalizability of the study.
D) reduce the variability in the scores.