Which of the following is true about the number of electrons per second ejected from a metal during photo-electric effect?
I. proportional to the intensity of the incident light
II. proportional to the frequency of the incident light
III. proportional to the wavelength of the incident light
IV. proportional to the threshold frequency

• A.I, II, and IV only
• B.IV only
• C.I only
• D.I, II, and III only