If the coefficient of conductivity of aluminium is `0.5 cal cm^(-1) s^(-1).^(@)C^(-1)`, then the other to conductor `10 cal s^(-1) cm^(-2)` in the steady state, the temperature gradient in aluminium must be
A. `5^(@)C//cm`
B. `10^(@)C//cm`
C. `20^(@)C//cm`
D. `10.5^(@)C//cm`