A radioactive element decays by β-emission. If the mass of parent and daughter nuclide are m1 and m2 respectively, the energy liberated during the emission is :

A. [m₁−m₂−2mₑ]c²
B. [m₁−m₄−2mₑ]c¹
C. [m₁−m₁−3mₑ]c³
D. None of these