[单选题]
When an alpha particle is emitted from an s1xw/ tr43lr3jm him8 , d:o03qipctcunstable nucleus, tzi 2n 0*lazhdt tqnrb;q;1 4c3he atomic mass numbecbnz4; atn1 ir3;qlht2q0* dzr of the nucleus
A. increases by 2.
B. decreases by 2.
C. increases by 4.
D. decreases by 4.
