A slab of thickness $L$, as shown in the figure below, has cross-sectional area $A$ and constant thermal conductivity $k. T_1$ and $T_2$ are the temperatures at $x=0$ and $x=L$, respectively. Which one of the following options is the $\text{CORRECT}$ expression of the thermal resistance for steady-state one-dimensional heat conduction?
- $\frac{L}{k A}$
- $\frac{k}{L A}$
- $\frac{k A\left(T_1-T_2\right)}{L}$
- $\frac{A}{L k}$