At a High Temperature, Carbon Dioxide Decomposes to Produce Carbon $\leftrightarrows$

At a high temperature, carbon dioxide decomposes to produce carbon monoxide and oxygen. Which expression corresponds to the equilibrium constant for the reaction written as
CO₂(g) $\leftrightarrows$ CO(g) +1/2 O₂(g) ?

A) $\frac { \left[ \mathrm { CO } _ { 2 } \right] } { [ \mathrm { CO } ] \left[ \mathrm { O } _ { 2 } \right] }$
B) $\frac { \left[ \mathrm { CO } _ { 3 } \right] \left[ \mathrm { O } _ { 2 } \right] } { \left[ \mathrm { CO } _ { 2 } \right] }$
C) $\frac { [ \mathrm { CO } ] \left[ \frac { 1 } { 2 } \mathrm { O } _ { 2 } \right] } { \left[ \mathrm { CO } _ { 2 } \right] }$
D) $\frac { [ \mathrm { CO } ] \left[ \mathrm { O } _ { 2 } \right] ^ { 1 / 2 } } { \left[ \mathrm { CO } _ { 2 } \right] }$
E) $\frac { \frac { 1 } { 2 } [ \mathrm { CO } ] \left[ \mathrm { O } _ { 2 } \right] } { \left[ \mathrm { CO } _ { 2 } \right] }$