The path difference between the two waves $y_{1}=a_{1} \sin \left(\omega t-\frac{2 \pi x}{\lambda}\right)$ and $y_{2}=a_{2} \cos \left(\omega t-\frac{2 \pi x}{\lambda}+\phi\right)$ is
(a) $\frac{\lambda}{2 \pi} \phi$
(b) $\frac{\lambda}{2 \pi}\left(\phi+\frac{\pi}{2}\right)$
(c) $\frac{2 \pi}{\lambda}\left(\phi-\frac{\pi}{2}\right)$
(d) $\frac{2 \pi}{\lambda}(\phi)$