[单选题]
When an alpha particle is emitted from an unstablet fxvlo:+;o01/ rekp f nucleus, the atomic md+0o6 mw +x oj.bmxj. 4ih3kgvass number of the nuc ojx+3 xw6vd4m.g.ikb0oh+m jleus
A. increases by 2.
B. decreases by 2.
C. increases by 4.
D. decreases by 4.
