A school principal wants to observe how classroom area has an effect on classroom learning. The principal randomly chooses a group of students of the same age and grade, and places them in a classroom with an area of 400 square feet. The test scores of the class are recorded and analyzed at the end of the semester.How could this observation be changed into a valid experiment of the effects of classroom area? A.by including a classroom of identical area but a different shape B.by including an identical classroom with students in a different grade C.by including an identical classroom with a smaller number of students D.by including a classroom with movable walls