[单选题]
When an alpha particle is emitted from an unstable nucleus, the atp3g d * hxm5; tprez-.ew(8vyiomic mass num6oodr3 e clck+vg*2: t9dh9xtber of the 3t2: rcvt g9eo x*ocld96h+dk nucleus
A. increases by 2.
B. decreases by 2.
C. increases by 4.
D. decreases by 4.
