21. Students in Mr. Clark's science class were trying to explain why we see the different phases of the moon. Which student's explanation is correct?
A. Student A explained that we see the different phases because the moon revolves around the earth.
B. Student C explained that we see the different phases because the moon revolves around the sun.
C. Student B explained that we see the different phases because the moon is very large. D. Student D explained that we see the different phases because the moon is covered with many craters.