The displacement $y$ of a particle in a medium can be expressed as, $y=10^{-6} \sin \left(100 t+20 x+\frac{\pi}{4}\right) m$ where
$\mathrm{t}$ is in second and $x$ in meter. The speed of the wave is $[2004]$
(A) $20 \mathrm{~m} / \mathrm{s}$
(B) $5 \mathrm{~m} / \mathrm{s}$
(C) $2000 \mathrm{~m} / \mathrm{s}$
(D) $5 \pi \mathrm{m} / \mathrm{s}$