If the threshold wavelength for a metal is $\lambda_0$ and the wavelength of incident light is $\lambda$,what will be the maximum velocity of the emitted electrons?

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