A tuning fork of frequency n is held near the open end of a tube which is closed at the other end and the length of the tube is adjusted until resonance occurs. If the two shortest lengths that produce resonance are ,l₁ and 1₂, the speed of the sound is

• A

  $\mathrm{n}\left({\mathrm{l}}_2−{\mathrm{l}}_1\right)$

• B

  $\mathrm{n}\left({\mathrm{l}}_2−{\mathrm{l}}_1\right)/2$

• C

  $2\mathrm{n}\left({\mathrm{l}}_2+{\mathrm{l}}_1\right)$

• D

  $2\mathrm{n}\left({\mathrm{l}}_2−{\mathrm{l}}_1\right)$