When an Unstable Nucleus Decays by Emitting an Alpha Particle $\mathrm { Z }$

When an unstable nucleus decays by emitting an alpha particle, the atomic number $\mathrm { Z }$ of the nucleus

A) decreases by 4 .
B) decreases by 2 .
C) increases by $4 .$
D) increases by 2 .
E) remains unchanged.