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) ${ }_{5} \mathrm{~B}^{10}$