A metal wire of linear mass density $9.8\text{g/m}$ is stretched with a tension of $10\text{kg-wt}$ between two rigid supports $1\text{metre}$ apart. The wire passes at its middle point between the poles of a magnet and it vibrates in resonance when carrying an alternating current of frequency $n$. The frequency $n$ of the alternating source is:

1. A
   $50\text{Hz}$
2. B
   $100\text{Hz}$
3. C
   $200\text{Hz}$
4. D
   $25\text{Hz}$