The equation represents the thermal decomposition of magnesium carbonate. $\text{MgCO}_3 \rightarrow \text{MgO} + \text{CO}_2$ $[M_r: \text{MgCO}_3, 84]$ What mass of magnesium oxide is produced when $21.0\,\text{g}$ of magnesium carbonate is fully decomposed?
- A$1.9\,\text{g}$
- B$4.0\,\text{g}$
- C$10.0\,\text{g}$
- D$40.0\,\text{g}$