The intensity of solar radiation varies across the earth's surface according to a seasonal cycle. What is the major cause of this variation? Select one: a. variations in the amount of radiation emitted by the sun during the course of the year b. variations in the distance between the earth and the sun during the course of the year c. the earth's daily rotation about its axis, leading to day and night d. the constant tilt of the earth's rotation axis in the same direction relative to the plane defined by the annual orbit of the earth around the sun