Photoelectric emission is observed from a metallic surface for frequencies $v_1$ and $v_2$ of the incident light rays $(v_1 > v_2)$. If the maximum values of kinetic energy of the photoelectrons emitted in the two cases are in the ratio of $1 : k$, then the threshold frequency of the metallic surface is :

Question

( A ) $\frac{kv_2 -v_1}{k-1}$
( B ) $\frac{kv_1 -v_2}{k-1}$
( C ) $\frac{v_2 -v_1}{k-1}$
( D ) $\frac{v_2 -v_1}{k}$