A resistance of $2 \Omega$ is connected across one gap of a meter-bridge and an unknown resistance, greater than $2 \Omega$, is connected across the other gap. When these resistances are interchanged, the balance point shifts by $20cm$. The unknown resistance is

Question

$\begin {array} {1 1} (A)\;3\Omega & \quad (B)\;4\Omega \\ (C)\;5\Omega & \quad (D)\;6\Omega \end {array}$