Photoelectric emission is observed from a surface for frequencies v1 and v2 of the incident radiation (v1>v2). If the maximum kinetic energies of the photoelectrons in two cases are in ratio 1 : K then the threshold frequency v0 is given by
(a) v2-v1/K-1
(b) Kv1-v2/K-1
(c) Kv2-v1/K-1
(d) v2-v1/K