1. Which among the following represent a set of isotopes? Atomic nuclei containing I. 20 protons and 20 neutrons. II. 21 protons and 19 neutrons. III. 22 neutrons and 18 protons. IV. 20 protons and 22 neutrons. V. 21 protons and 20 neutrons. (A) I, II, III ; (B) III, IV ; (C) I, V ; (D) I, IV and II, V ; (E) No isotopes are indicated.