In the Franck-Hertz Experiment on Potassium Shows a Threshold Occurs

In the Franck-Hertz experiment on potassium shows a threshold occurs at 1.55 eV, what which of the following wavelengths would you expect to observe just above this threshold?

A) 155 nm
B) 374 nm
C) 400 nm
D) 600 nm
E) 800 nm