Two adjacent piano keys are struck simultaneously. The notes emitted by them have frequencies n₁ and n₂. The number of beats heard per second is

• A

  $\frac{1}{2}({\mathrm{n}}_1-{\mathrm{n}}_2)$

• B

  $\frac{1}{2}({\mathrm{n}}_1+{\mathrm{n}}_2)$

• C

  ${\mathrm{n}}_1~{\mathrm{n}}_2$

• D

  $2({\mathrm{n}}_1-{\mathrm{n}}_2)$