constant pressure question?

At a total pressure of 1.0 atm, dinitrogen tetraoxide is 50% dissociated at a temperature of 60 degree C, according to the following equation. N2O4 ==> 2NO2 What is the value of the equilibrium constant, Kp, for this reaction at 60 degree C? A) 1/3 atm B) 2/3atm C) 4/3atm D) 2atm Answer is 3 but how is the question??