An open pipe and a closed pipe have fundamental frequencies $n_1$ and $n_2$ respectively. They are combined to form a closed pipe. Fundamental frequency of the combined pipe is

• A

  $\mathrm{n}_1+\mathrm{n}_2$
• B

  $\mathrm{n}_1-\mathrm{n}_2$
• C

  $\frac{\mathrm{n}_1 \mathrm{n}_2}{\mathrm{n}_1-2 \mathrm{n}_2}$
• Correct answer
  D

  $\frac{\mathrm{n}_1 \mathrm{n}_2}{\mathrm{n}_1+2 \mathrm{n}_2}$

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