A string is divided into three parts having lengths $\ell_{1}, \ell_{2}$ and $\ell_{3}$ each. If the fundamental frequency of these parts are $\mathrm{f}_{1}, \mathrm{f}_{2}$ and $\mathrm{f}_{3}$ respectively, then the fundamental frequency of the original string $\mathrm{f}=\ldots \ldots \ldots$
(A) $\sqrt{f}=\sqrt{f}_{1}+\sqrt{f}_{2}+\sqrt{f}_{3}$
(B) $\mathrm{f}=\mathrm{f}_{1}+\mathrm{f}_{2}+\mathrm{f}_{3}$
(C) $(1 / \mathrm{f})=\left(1 / \mathrm{f}_{1}\right)+\left(1 / \mathrm{f}_{2}\right)+\left(1 / \mathrm{f}_{3}\right)$
(D) $(1 / \sqrt{f})=\left(1 / \sqrt{f}_{1}\right)+\left(1 / \sqrt{f}_{2}\right)+\left(1 / \sqrt{f}_{2}\right)$