An unknown stable nuclide after absorbing a neutron emits an electron, and the new nuclide splits spontaneously into two alpha particles. The unknown nuclide is :
(A) ${ }_{4} \mathrm{Be}^{9}$
(B) ${ }_{3} \mathrm{Li}^{7}$
(C) ${ }_{2} \mathrm{He}^{4}$
(D) ${ }_{5} \mathrm{~B}^{10}$