A string is stretched between fixed points separated by 75 $\mathrm{cm} .$ It is observed to have a resonant frequencies of $420 \mathrm{~Hz}$ and $315 \mathrm{~Hz}$. There are other resonant frequencies between these two. Then the lowest frequency for this string is $\ldots \ldots \ldots \ldots . . . h z$
(A) $1.05$
(B) 1050
(C) $10.5$
(D) 105