Which of the Following Statements Is True

Which of the following statements is true?

A) Given the atomic number of an element, the atomic mass of the element can be calculated
B) An isotope is identified by its mass number
C) The number of neutrons in an atom determines the chemical reactivity of that isotope
D) The total contribution to the mass of an atom from protons and electrons varies among the atoms of an element
E) Mass differences among atoms of an element cannot be accounted for with our current understanding of the atom