[单选题]
When an alpha particle is emitted from an unst jh4(rxz: r ael4bg:qmm* y4d2ajf0go avrr5;v .b-67yk rvo/mble nucleus, the atomic mass number of theb mo7k /f-6rrvga5vvj y;o .0r nucleus
A. increases by 2.
B. decreases by 2.
C. increases by 4.
D. decreases by 4.
