A nucleus of an element that is unstable decays by releasing either an $\alpha$-particle or a $\beta^-$-particle, forming a nucleus of another element. That nucleus is unstable too and releases an $\alpha$-particle or a $\beta^-$-particle. This sequence carries on until an isotope of the original element is formed. What is the smallest possible number of these particles emitted?
- A$2$
- B$3$
- C$4$
- D$5$