Goku is studying the affect of exposure to sunlight on mood. He has one group of participants work for eight hours a day in an office with a window, and another group work for eight hours a day a in an office without a window. After one week, Goku measures participants' mood using the positive and negative affect scale (PANAS).
In this study, Goku _______ exposure to sunlight.
a. measured
b. manipulated
c. mediated