A spherical source of power 4 W and frequency 800 Hz is emitting sound waves. The intensity of waves at a distance 200 m is

• A

  $8\times {10}^{-6}\mathrm{W}/{\mathrm{m}}^2$

• B

  $2\times {10}^{-4}\mathrm{W}/{\mathrm{m}}^2$

• C

  $1\times {10}^{-4}\mathrm{W}/{\mathrm{m}}^2$

• D

  $4\mathrm{W}/{\mathrm{m}}^2$