When a metallic surface is illuminated with radiation of wavelength \[\lambda \], the stopping potential is V. If the same surface is illuminated with radiation of wavelength \[2\lambda \] the stopping potential is \[\frac{V}{4}.\]The threshold wavelength for the metallic surface is:

• A. \[4\lambda \]     
• B. \[5\lambda \]
• C. \[\frac{5}{2}\lambda \]
• D. \[3\lambda \]