A nucleus of an element that is unstable decays by releasing either an $\alpha$-particle or a $\beta^-$ particle, forming a nucleus of a different element. That nucleus is also unstable and likewise releases either an $\alpha$-particle or a $\beta^-$ particle. This sequence goes on until an isotope of the original element is formed. What is the least possible number of these particles emitted?
- A2
- B3
- C4
- D5