When a metal surface is irradiated with light of frequency $x \ Hz$,the kinetic energy of emitted photoelectrons is $z \ J$. When the same metal is irradiated with light of frequency $y \ Hz$,the kinetic energy of emitted electrons is $\frac{z}{3} \ J$. What is the threshold frequency (in $Hz$) of the metal?

• A
  $\frac{3x-y}{2}$
• B
  $\frac{3y-x}{2}$
• C
  $\frac{2y-x}{3}$
• D
  $\frac{2}{3}(y-x)$