14．Which of the following are true for electrons? (A)The same quantum state cannot be occupied by more than one electron. (B)Two electrons can have the same spin state. (C)At any temperature, electrons in a solid cannot be found above the Fermi level EF. (D)The existence of energy gap in a semiconductor can be proved within quantum physics. (E)None of the above.

9．Which are the following statements correct? (A)The maximum kinetic energy of the recoil electron in Compton scattering is given by ,where f is the frequency of the incident photon, h the Plank constant, c speed of the light and m rest mass of an electron. (B)The black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature. (C)The photon is massless so that the gravitational force will not affect its frequency as it leaves a star. (D)The maximum kinetic energy of the emitted photoelectron depends on the frequency of the incident light. (E)x-rays were found to be unaffected by electric and magnetic fields, to pass readily through opaque materials and to cause phosphorescent to glow.

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