Two sound sources produce progressive waves given by ${\mathrm{y}}_1=12\cos 100\mathrm{\pi t}$ and ${\mathrm{y}}_2=12\cos 102\mathrm{\pi t}$ near the ear of an observer. When sounded together, the observer will hear beats. The observed beat frequency (in Hz) is

• A

  1 Hz

• B

  2 Hz

• C

  3 Hz

• D

  4 Hz