[单选题]
When a neutron is emitted fropsm5ct : )s .bile+fuas)x 3:mm an unstable nuc5 6rogm hre.gwu+0t) tleus, the atomic mass number of the nucleustr+ t5mgh6g uo )wre0.
A. increases by 1.
B. decreases by 1.
C. does not change.
D. none of the given answers
