Which of the following statements is correct?
- ADoctors can use the very insoluble $\text{MgSO}_4$ to investigate the digestive system.
- BFarmers can lower the pH of soil by spreading $\text{CaCO}_3$ on it.
- CStudents can test a solution for $\text{SO}_4^{2-}$ ions by using $\text{Ba(NO}_3)_2(aq)$ followed by $\text{HNO}_3(aq)$.
- DThe insoluble hydroxide, $\text{Ba(OH)}_2$, can be safely used to lower the acidity of the stomach.