If $\lambda_0$ and $\lambda$ are respectively the threshold wavelength and wavelength of incident light,the velocity of photoelectrons ejected from the metal surface is

• A
  $\sqrt{\frac{2h}{m}(\lambda_0-\lambda)}$
• B
  $\sqrt{\frac{2hc}{m}(\frac{\lambda_0-\lambda}{\lambda\lambda_0})}$
• C
  $\sqrt{\frac{2hc}{m}(\lambda_0-\lambda)}$
• D
  $\sqrt{\frac{2hc}{m}(\frac{1}{\lambda_0}-\frac{1}{\lambda})}$