The path difference between two waves given by the equations $y_1 = a_1 \sin \left(\omega t - \frac{2 \pi x}{\lambda}\right)$ and $y_2 = a_2 \sin \left(\omega t - \frac{2 \pi x}{\lambda} + \phi\right)$ is

• A
  $\left(\frac{\lambda}{\pi} \phi\right)$
• B
  $\frac{\lambda}{\pi} \left(\phi - \frac{\pi}{2}\right)$
• C
  $\frac{\lambda}{2 \pi} \phi$
• D
  $\frac{\lambda}{2 \pi} \left(\phi - \frac{\pi}{2}\right)$