15. In X-ray spectrum, which of the following statements is correct?
(A) The electron beam is accelerated in the air to strike a target and generate X-rays.
(B) The minimum frequency of X-ray is proportional to the potential difference between anode and cathode.
(C) Most of the generated X-rays are bremsstrahlung radiation.
(D) Characteristic X-rays are the radiation generated by atom nucleus.
(E) Most kinetic energy of electrons is converted to X-rays.