Two reactions, X and Y, form carbon dioxide. $\text{CH}_4 \xrightarrow{X} \text{CO}_2 \xleftarrow{Y} \text{CaCO}_3$ What kinds of reaction are X and Y?
- AX: combustion; Y: combustion
- BX: combustion; Y: thermal decomposition
- CX: thermal decomposition; Y: combustion
- DX: thermal decomposition; Y: thermal decomposition