The displacement y of a particle in a medium can be expressed as: $\mathrm{y}={10}^{-6}\sin (100\mathrm{t}+20\mathrm{x}+\mathrm{\pi }/4)\mathrm{m},$ 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

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