21. The reaction N2(g) + 3H2(g) 2NH3(g) takes place in a 1.0-L vessel initially filled with an equimolar mixture of N2 and H2 at a pressure of 12.0 atm. When equilibrium is reached, the partial pressure of H2 is 3.0 atm. Determine the value of Kp for this reaction. (A) 0.915 (B) 0.333 (C) 0.133 (D) 0.030