Photoelectric emission is observed from a metallic surface for frequencies ${\mathrm{\nu }}_1 \mathrm{and} {\mathrm{\nu }}_2$ of the incident light rays (${\mathrm{\nu }}_1>{\mathrm{\nu }}_2$). If the maximum values of kinetic energy of the photoelectrons emitted in the two cases are in the ratio of I : k, then the threshold frequency of the metallic surface is

• A

  $\frac{{\mathrm{v}}_1-{\mathrm{v}}_2}{\mathrm{k}-1}$

• B

  $\frac{{\mathrm{kv}}_1-{\mathrm{v}}_2}{\mathrm{k}-1}$

• C

  $\frac{{\mathrm{kv}}_2-{\mathrm{v}}_1}{\mathrm{k}-1}$

• D

  $\frac{{\mathrm{v}}_2-{\mathrm{v}}_1}{\mathrm{k}}$