Which of the following is true about the photoelectric effect? a) The number of electrons emitted increases with intensity of light, regardless of the frequency. b) The number of electrons emitted increases with the intensity of light, but only if the frequency of light is above the threshold frequency. c) The number of electrons emitted increases with the intensity of light, but only if the frequency of light is below the threshold frequency. d) The kinetic energy of the emitted electrons depends only on the intensity of the light.