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

• Correct answer
  A

  $50 \mathrm{~Hz}$
• B

  $100 \mathrm{~Hz}$
• C

  $200 \mathrm{~Hz}$
• D

  $25 \mathrm{~Hz}$

Hence, the correct answer is option: 'A'.