When a certain metal surface is illuminated with light of frequency $v$,the stopping potential for photoelectric current is $V_0$. When the same surface is illuminated by light of frequency $\frac{v}{2}$,the stopping potential is $\frac{V_0}{4}$,the threshold frequency of photoelectric emission is

• A
  $\frac{v}{6}$
• B
  $\frac{v}{3}$
• C
  $\frac{2 v}{3}$
• D
  $\frac{4 v}{3}$