In a wave, the path difference corresponding to a phase difference of $\mathrm{\varphi }$ is 

• A

  $\frac{\mathrm{\pi }}{2\mathrm{\lambda }}\mathrm{\varphi }$

• B

  $\frac{\mathrm{\pi }}{\mathrm{\lambda }}\mathrm{\varphi }$

• C

  $\frac{\mathrm{\lambda }}{2\mathrm{\pi }}\mathrm{\varphi }$

• D

  $\frac{\mathrm{\lambda }}{\mathrm{\pi }}\mathrm{\varphi }$