The force per unit length of wire $\mathrm{B}$, exerted by wire $\mathrm{A}$ is given by $\mathrm{F}=\frac{\mathrm{kl}_{1} \mathrm{I}_{2}}{\mathrm{r}} .$ Where $\mathrm{r}$ is the separation between the parallel wires carrying currents $\mathrm{I}_{1}$ and $\mathrm{I}_{2} .$ The dimensional formula of $\mathrm{k}$ is
(a) $\mathrm{ML}^{2} \mathrm{~T}^{-2} \mathrm{I}^{2}$
(b) $\mathrm{MLT}^{-2} \mathrm{I}^{-2}$
(c) $\mathrm{ML}^{2} \mathrm{TI}^{2}$
(d) $\mathrm{M}^{2} \mathrm{~L}^{2} \mathrm{~T}^{2} \mathrm{I}^{-2}$