c
for first resonance
$\ell_{1}+\varepsilon=\frac{v}{4 \mathrm{f}_{0}}$
for second resonance
$\ell_{2}+\varepsilon=\frac{3 \mathrm{v}}{4 \mathrm{f}_{0}}$
$\ell_{3}+\varepsilon=\frac{5 \mathrm{v}}{4 \mathrm{f}_{0}}$
Solving get $\ell_{3}=2 \ell_{2}-\ell_{1}$