[单选题]
When a neutron is emix zmvydp(a-53zbo fb7j 6*l6mttmlvka( we q0n2,a sn4 lqx.5b(ed from an unstable nucleus, the atomic mass number of the nuclnm 4elxabq.52sqa k (w,ln(v0eus
A. increases by 1.
B. decreases by 1.
C. does not change.
D. none of the given answers
