In a Rutherford scattering experiment when a projectile of charge $Z_1$ and mass $M_1$ approaches a target nucleus of charge $Z_2$ and mass $M_2$, the distance of closest approach is $r_0$. The energy of the projectile is :

Question

( A ) Directly proportional to ${Z_1} {Z_2}$
( B ) Directly proportional to ${M_1}\times {M_2}$
( C ) Directly proportional to mass ${M_1}$
( D ) Inversely proportional to ${Z_1}$