The threshold frequency for a certain metal is $v_0$. When radiation of frequency $2v_0$ is incident on this metal surface,the maximum velocity of the emitted photoelectrons is $2 \times 10^6 \ m/s$. If radiation of frequency $3v_0$ is incident on the same metal surface,the maximum velocity of the emitted photoelectrons is . . . . . .

• A
  $\sqrt{2} \times 10^6 \ m/s$
• B
  $2\sqrt{2} \times 10^6 \ m/s$
• C
  $4 \times 10^6 \ m/s$
• D
  $5 \times 10^6 \ m/s$