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. If the thermal conductivities of rods $A$ and $B$ are $3K$ and $K$ respectively,and $T_A$ and $T_B$ are the temperature drops across rods $A$ and $B$,then:

• A
  $\frac{T_A}{T_B} = \frac{3}{1}$
• B
  $\frac{T_A}{T_B} = \frac{1}{3}$
• C
  $\frac{T_A}{T_B} = \frac{3}{4}$
• D
  $\frac{T_A}{T_B} = \frac{4}{3}$