A rod of length l and cross sectional area A has a variable conductivity given by `K=alphaT`, where `alpha` is a positive constant T is temperatures in Kelvin. Two ends of the rod are maintained at temperatures `T_1` and `T_2(T_1gtT_2)`. Heat current flowing through the rod will be
A. `(A alpha(T_(1)^(2)-T_(2)^(2)))/l`
B. `(A alpha(T_(1)^(2)-T_(2)^(2)))/l`
C. `(A alpha(T_(1)^(2)+T_(2)^(2)))/(3l)`
D. `(A alpha(T_(1)^(2)-T_(2)^(2)))/(2l)`