20. Three different monochromatic light sources are used to liberate free electrons from the surface of an unknown material. Source 1 has wavelength 500 nm, and a stopping potential of 1.08 volts is observed. Source 2 has wavelength 650 nm and a stopping potential of 0.510 volts. If source 3 has wavelength 590 nm, what is the maximum energy of electrons that can be liberated from the surface by source 3? Answer in electron volts. (A) 0.50 (B) 0.70 (C) 0.20 (D) 0.90 (E) 0.10