When a metallic surface is illuminated with monochromatic light of wavelength$\lambda$ stopping potential for photoelectric current is $3V_0$. When the   same metallic surface is illuminated with a light of wavelength $2\lambda$ the stopping potential is $V_0$. The threshold wavelength for the surface is

• A

  $6 \lambda$

• B

  $4 \lambda$

• C

  $\frac{4\lambda }{3}$

• D

  $8 \lambda$