If $E$ is the Emerson effect,$x$ is the rate of oxygen production in a mixed beam of light,$y$ is the rate of $O_2$ production in low wavelength light (short wavelength),and $z$ is the rate of $O_2$ evolution in high wavelength red light,what will be the value of $E$?

• A
  $E = \frac{x - z}{y}$
• B
  $E = \frac{x - y}{z}$
• C
  $E = \frac{x}{y + z}$
• D
  $E = \frac{x}{y - z}$