Light of different frequencies and intensities falls on a clean metal surface. Which of the statements about the photoelectric effect are correct?

• Statement I: Increasing the intensity of light of a fixed frequency increases the kinetic energy of the ejected electrons.
• Statement II: Light below the threshold frequency ejects no electrons, however intense it is.
• Statement III: Above the threshold frequency, the number of electrons ejected per second rises with the intensity of light.
• Statement IV: Electrons are ejected as soon as suitable light strikes the metal, with no measurable time lag.