The maximum velocity of an electron emitted by light of wavelength $\lambda$ incident on the surface of a metal of work function $\phi$ is (Where $h =$ Planck's constant,$m =$ mass of electron,and $c =$ speed of light).

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