The Four Seasons-Spring, Summer, Fall, and Winter-Are Caused by the Sun's

The four seasons-spring, summer, fall, and winter-are caused by the Sun's changing its daily altitude over a period of 1 year. The Sun's altitude changes

A) simply because Earth revolves eastward about the Sun.
B) simply because Earth rotates on its axis.
C) simply because Earth's axis is tilted 23.5° from the normal to the ecliptic plane.
D) because of the combined effects of (A) and (C) .