The number of protons, neutrons and electrons in species A to E are given in the following table :

Indicate from the above table the species that represent a pair of isotopes.

Explanation

Those species which contain the same number of protons but different number of neutrons will be a pair of isotopes. In the above table only two species C and E have the same number of protons (17 each) but different number of neutrons (20 and 18 respectively). Thus, C and E are a pair of isotopes. (Please note that the atomic number 17 is of chlorine, so C and E are actually the two actually the two isotopes of chlorine).