Which of the Following Contribute to the Variable Cost of Generating

Which of the following contribute to the variable cost of generating electricity from solar panels?
(i) Sunshine.
(ii) Price of coal.
(iii) Purchase cost of solar panels.
(iv) Maintenance expenses of panels.

A) (i) .
B) (ii) .
C) (iii) .
D) (iv) .
E) (i) and (iv) .