Formic acid is heated with conc. $\text{H}_2\text{SO}_4$ at $100^\circ\text{C}$ to form $A$ and $B$. When $\text{Fe}_2\text{O}_3$ is heated strongly with $B$, the products formed are $C$ and $D$. $C$ can also be obtained by reacting $\text{CaCO}_3$ with dil. HCl. What is $D$?