A school principle wants to observe if natural light has an effect in the classroom learning. The principle randomly chooses two groups of students of the same age and grade, and places both groups in rooms on opposite sides of the school, one facing east and one facing west, during a 10am class. The test scores of both classes are compared at the end of the semester. How could this observation be changed into a valid experiment of the effects of natural light?
A. by introducing adjustable light fixtures in each classroom
B. by tinting the windows in both classrooms
C. by including classrooms facing north and south
D. by introducing adjustable blinds in each classrooms