Let $x$ be the mass number of $A$and $y$the atomic number. Then , since atomic number and mass number remain conserved, we have

${ }_y{A}^x\to { }_{y-2}{B}^{x-4} + { }_2\mathrm{H}{\mathrm{e}}^4$

${ }_{y-2}{B}^{x-4}\to { }_y{C}^{x-4} + 2{ }_{-1}{e}^0$

Hence, we observe that $A$ and $C$ are isotopes as their atomic numbers are same but mass numbers are different.