a
Initially $\frac{\mathrm{R}_{1}}{\mathrm{R}_{2}}=\frac{2}{3}=\frac{\ell}{100-\ell} \Rightarrow \ell=40 \mathrm{\,cm}$
Now $\frac{\mathrm{R}_{2}}{\mathrm{R}_{1}}=\frac{3}{2}=\frac{\ell}{100-\ell} \Rightarrow \ell=60 \mathrm{\,cm}$
shift $=20 \mathrm{\,cm}$