Why are the figures on the sistine chapel ceiling increase in size as one walks from the entrance to the high altar?

The art of using and understanding space involves a. varying the size of objects or subjects, so that some are smaller than others and the illusion of depth is created. b. showing a change from light to dark in an object by darkening areas that would have a shadow. c. placing part of one shape or form in front of another. d. all of the above.