Energy Levels Questions

Energy levels A, B and C of a certain atom correspond to increasing values of energy i.e., EA < EB < EC. If λ1, λ2 and λ3 are wavelengths of radiations corresponding to transitions C to B, B to A and C to A respectively, which of the following relations is correct ?

Monochromatic radiation emitted when electron on hydrogen atom jumps from first excited to the ground state irradiates a photosensitive material. The stopping potential is measured to be $3.57\text{V}$. The threshold frequency of the material is: