The three statements give possible reasons why an average value should be used for the solar constant.

I.   The Sun’s output varies during its 11 year cycle.
II.  The Earth is in elliptical orbit around the Sun.
III. The plane of the Earth’s spin on its axis is tilted to the plane of its orbit about the Sun.

Which are the correct reasons for using an average value for the solar constant?

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

A