Introduction for Reactions Practice Test

A student is checking whether $\text{Al}(s)+\text{HCl}(aq)\rightarrow \text{AlCl}_3(aq)+\text{H}_2(g)$ obeys conservation of atoms. Which statement describes the correct criterion for deciding if the equation is balanced?

A reaction is written as $\text{CaCO}_3(s)\rightarrow \text{CaO}(s)+\text{CO}_2(g)$. Without doing any extended calculations, which statement best explains why this equation already satisfies conservation of atoms?