When a certain metal was irradiated with light of frequency $4.0 \times 10^{16} \ s^{-1}$,the photoelectrons emitted had four times the kinetic energy as the kinetic energy of photoelectrons emitted when the same metal was irradiated with light of frequency $2.0 \times 10^{16} \ s^{-1}$. The threshold frequency $(v_0)$ of the metal in $s^{-1}$ is

• A
  $2 \times 10^{16}$
• B
  $4 \times 10^{16}$
• C
  $2.5 \times 10^{16}$
• D
  $1.33 \times 10^{16}$