a
Internal resistance $r=\frac{R\left[l-l^{\prime}\right]}{l^{\prime}}$
where $l=$ balanced length with key open.
$l^{\prime}=$ balanced length with key $K$ closed
$\therefore r=\frac{132.40[70-60]}{60}=\frac{132.40 \times 10}{60}$
$=21.06\, \Omega \approx 22.1\, \Omega$