Two rods $A$ and $B$ of the same cross-sectional area $A$ and length $l$ are connected in series between a source $(T_1 = 100^{\circ}C)$ and a sink $(T_2 = 0^{\circ}C)$ as shown in the figure. The rods are laterally insulated. The thermal conductivities of rods $A$ and $B$ are $3K$ and $K$ respectively. The ratio of the thermal resistance of rod $A$ to that of rod $B$ is:
- A$\frac{R_A}{R_B} = \frac{1}{3}$
- B$\frac{R_A}{R_B} = 3$
- C$\frac{R_A}{R_B} = \frac{3}{4}$
- D$\frac{R_A}{R_B} = \frac{4}{3}$
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