The displacement y of a particle in a medium can be expressed as : $\mathrm{y}= {10}^{-6} \sin [100\mathrm{t}+20\mathrm{x} + \frac{\mathrm{\pi }}{4}] \mathrm{m}$, where t is in second and x in meter. The speed of wave is

• A

  2000 m/s

• B

  5 m/s

• C

  20 m/s

• D

  5$\mathrm{\pi } \mathrm{m}/\mathrm{s}$