Red light (v = 4.3 to 4.6 ×1014Hz) of any brightness may shine on a piece of weak yellow light shines on potassium metal, photoelectrons are ejected. Select the suitable reason out of the following

Potassium has only one electron in its valence shell 3s1. The threshold frequency of potassium is higher than frequency of red light but lower than that of yellow light. Hence, yellow light causes photoelectron emission from potassium and not the red light.