Staements I: `._zX^4` undergoes `2 alpha`-decays, `2 beta`-decays (negative `beta`) and `2 gamma`-decays. As a result, the daughter product is `._z-2X^(A-B)`.
Staements II: In `beta`-decay, the mass number decreases by 4 unit and atomic number decreases by `2` unit. In `beta`-decay (negative `beta`), the mass number remains unchanged and atomic number increases by `1` unit. In `gamma`-decay, mass number and atomic number remain unchanged.
