Four substances, P, Q, R and S, are examined as shown. Identify P, Q, R and S.
- AP: Mg, Q: $\text{Na}_2\text{CO}_3$, R: $\text{NH}_4\text{Cl}$, S: NaCl
- BP: Mg, Q: $\text{NH}_4\text{Cl}$, R: $\text{Na}_2\text{CO}_3$, S: NaCl
- CP: Mg, Q: $\text{Na}_2\text{CO}_3$, R: NaCl, S: $\text{NH}_4\text{Cl}$
- DP: $\text{Na}_2\text{CO}_3$, Q: Mg, R: NaCl, S: $\text{NH}_4\text{Cl}$