Which reaction has an equilibrium constant, $K_p$, with no units?
- A$\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g)$
- B$3\text{H}_2(g) + \text{N}_2(g) \rightleftharpoons 2\text{NH}_3(g)$
- C$2\text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g)$
- D$2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$