[单选题]
When an alpha particle is emitted fsjbnmtej(l rwl5yj4089z- p*rokdvypq r dm.j/eu/.1nh91 h+ym an unstable nucleus, the atomic mass number oymdrhj9p+ de/n/. h1 yk1vqu.f the nucleus
A. increases by 2.
B. decreases by 2.
C. increases by 4.
D. decreases by 4.
