The displacement $y$ of a particle in a medium can be expressed as $y = 10^{-6} \sin (110 t + 20 x + \pi/4)\;m$ where $t$ is in seconds and $x$ in meter. The speed of the wave is
( A ) $2000\; m/s$
( B ) $5\; m/s$
( C ) $5 \pi\; m/s$
( D ) $20\;m/s$