An Atom of the Isotope ¹³⁷Ba Consists of How Many

An atom of the isotope ¹³⁷Ba consists of how many protons (p) , neutrons (n) , and electrons (e)

A) 56 p, 137 n, 56 e
B) 56 p, 81 n, 56 e
C) 137 p, 81 n, 56 e
D) 56 p, 56 n, 56 e
E) 81 p, 56 n, 81 e