An electron of mass $'m'$ moving with velocity $'v'$ collids head on with an atom of mass M. As a result of the collision a certain fixed amount of energy $\Delta E$ is stored internally in the atom. The minimum initial velocity possessed by the electron is

Question

$a)\;\sqrt {\large\frac{2(M+m) \Delta E}{Mm}}$

$b)\;\sqrt {\large\frac{2(M-m) \Delta E}{Mm}}$

$c)\;\sqrt {\large\frac{2M \Delta E}{Mm}}$

$d)\;zero$