What is the resultant wave obtained for $\phi=\frac{\pi}{2}$ rad when two harmonic waves are $y_{1}(x, t)=0.2 \sin (x-3 t)$
$y_{2}(x, t)=0.2 \sin (x-3 t+\phi) ?$
(a) $y=0.28 \sin \left(x-3 t+\frac{\pi}{4}\right)$ (b) $y=3 \sin (x-3 t)$
(c) $y=0.28 \sin \left(x-3 t-\frac{\pi}{4}\right)\left(\right.$ d) $y=0.28 \sin \left(x+3 t-\frac{\pi}{4}\right)$