VERY URGENT - 100 points

Which of the following equations best describes heat conduction through a single sheet of material?
A. ∅=k(T2−T1) / L
B. ∅=k(T1+T2) / L
C. ∅=k(T1−T2)(T1+T2) / L
D. ∅=k(T1−T2) / L

give simple answer PLS