SAT writing/choose the sentence that sounds best question?

Question

The radiation emanating from the Sun has been very nearly constant for millions of years, but the amount of heat that falls on Earth's surface [depends on both the length of the period of daylight as well as on] the angle of incidence of incoming sunlight.

A. depends on both the length of the period of daylight as well as on

B. depends both on the length of the period of daylight as well as

C. depends both on the length of the period of daylight and on (correct answer)

D. both depends on the length of the period of daylight and

E. both depends on the length of the period of daylight and on

Can someone explain why "as well as" would not work?

Answer 1

You don't use "both" with "as well as".

Answer 2

If you were using "as well as" at the end, you would not use the "both" at the beginning.  Using both (of the phrases together) is redundant (unnecessarily repetitive).