question: Q:Light of variable frequency shines on the metal surface of a photoelectric tube. Einstein’s theory of the photoelectric effect predicts that the? 
  A:work function of the metal is proportional to the frequency;B:work function of the metal is proportional to the wavelength;C:current in the tube is a linear function of the wavelength;D:potential difference necessary to stop the emitted electrons is a linear function of the frequency above the threshold frequency