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6.) Which are isotopes? An atom that has an atomic number of 20 and a mass number of 42 is an isotope of an atom that has A. 22 protons and 20 neutrons. B. an atomic number of 21 and a mass number of 42. C. 22 neutrons and 20 protons. D. an atomic number of 20 and a mass number of 40. E. none of the above