A nitrogen-hydrogen mixture, which starts with a mole ratio of $1:3$, comes to equilibrium with ammonia after $50\%$ of the nitrogen has reacted. The total pressure at equilibrium is $p$. $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$ What is the partial pressure of ammonia in the equilibrium mixture?
- A$\frac{p}{6}$
- B$\frac{p}{4}$
- C$\frac{p}{3}$
- D$\frac{p}{2}$