The equation of a stationary wave is $y=0.8 \cos \left(\frac{\pi x}{20}\right) \sin 200 \pi t$ where $x$ is in $c m .$ and $t$ is in $\mathrm{sec}$. The separation between consecutive nodes will be
(a) $20 \mathrm{~cm}$
(b) $10 \mathrm{~cm}$
(c) $40 \mathrm{~cm}$
(d) $30 \mathrm{~cm}$