A string is stretched between fixed points separated by $75\text{cm}$. It is observed to have resonant frequencies of $420\text{Hz}$ and $315\text{Hz}$. There are no other resonant frequencies between these two. Then, the lowest resonant frequency for this string is:

1. A
   $10.5\text{Hz}$
2. B
   $105\text{Hz}$
3. C
   $1.05\text{Hz}$
4. D
   $1050\text{Hz}$