The ratio of initial to final current through the battery when the switch is closed in the figure is

Two coils $A$ and $B$ are wound on the same iron core as shown in figure. The number of turns in the coil $A$ and $B$ are $N_A$ and $N_B$ respectively. Identify the correct statement

A small square loop of wire of side $l$ is placed inside a large square loop of wire $L\left(L\gg l\right)$. Both loops are coplanar and their centres coincide at point $O$ as shown in figure. The mutual inductance of the system is