The isotope 125B having a mass 12.014 u undergoes β− decay to 126C. 126C has an excited state of the nucleus (126C∗) at 4.041 MeV above its ground state. If 125B decay to 126C∗, the maximum kinetic energy of the β− particle in units of MeV nearly is
Take : m(126C)=12.0 u(1u=931.5 MeV/c2,Where c is the speed of light in vacuum)