[单选题]
When an alpha particle zq0qi,:shygzcl 0d0/ is emitted njd4 xqy 4zmpai)jq 9x4w*:b;*mo 0mzaz)od5from an unstable nucleus, the atomic mass number of the nucle )i aw5mbjaj0)zmdy4n ;xqd pq4*9*z4moz o:xus
A. increases by 2.
B. decreases by 2.
C. increases by 4.
D. decreases by 4.
