Isotopes of the same element must also have the same?

a) number of alpha particles
b) average mass number
c) atomic number
d) number of neutrons

The correct answer is c atomic number

Explanation:

When two atoms contain same atomic number or proton number then these two atoms are called isotopes of each other.

  For example protium and tritium are isotopes of each other as they contain same atomic number that is 1.

i think that the answer is c

the awnser is group 12