The displacement $y$ of a particle in a medium can be expressed as $y=10^{-6}\sin (100t+20x+\pi /4)\text{m}$, where $t$ is in seconds and $x$ in meter. The speed of the wave is:

1. A
   $2000\text{m/s}$
2. B
   $5\text{m/s}$
3. C
   $20\text{m/s}$
4. D
   $5\pi \text{m/s}$