If $n_1$, $n_2$ and $n_3$ are the fundamental frequencies of three segments into which a string is divided, then the original fundamental frequency n of the string is given by:

Question

( A ) $n = n_1 + n_2 + n_3$
( B ) $\frac{1}{n} = \frac{1}{n_1} + \frac{1}{n_2} + \frac{1}{n_3}$
( C ) ${\sqrt n} =\sqrt {n_1} + \sqrt{ n_2} + \sqrt{ n_3}$
( D ) $\frac{1}{\sqrt n} = \frac{1}{\sqrt {n_1}} + \frac{1}{\sqrt{ n_2}} + \frac{1}{\sqrt{ n_3}}$