A wave is represented by the equation $y=7 \sin \left(7 \pi t-0.04 x+\frac{\pi}{3}\right)$
where, $x$ is in metres and $t$ in seconds. The speed of the waves is
(a) $(175 \pi) \mathrm{m} / \mathrm{s}$
(b) $(49 \pi) \mathrm{m} / \mathrm{s}$
(c) $(49 / \pi) \mathrm{m} / \mathrm{s}$
(d) $(0.28 \pi) \mathrm{m} / \mathrm{s}$