[单选题]
When a neutron is emi f34uj6jalvw0go ce:4/ve(e ;c sy xc0tted from an unstable nucleus, the atoe*ejjcv+ f9o, nc, 5log8e.xhmic mass number of the n*o. cveex,je,cg8l9ojfh5 n+ucleus
A. increases by 1.
B. decreases by 1.
C. does not change.
D. none of the given answers
