Stress can be measured by the amount of a certain chemical found in saliva. Subjects must be sure to get saliva samples twice a
day at least 1 hour after eating or brushing teeth. In a study of 100 subjects, researchers tried to find a correlation between stress
levels and the wake up time of the individual. Which factor would NOT be an effective control that experimenters should add to
the experiment?

A) subjects are given the same diet
B) subjects are selected in similar age groups
C) subjects are selected with the same occupation
D) subjects are awakened at the same time everyday