Which statement about an element’s average atomic mass is correct? a. it is determined by counting the number of isotopes in a sample of the element. b. it is equal to one-twelfth the mass of the most common isotope. c. it is a weighted average, so common isotopes have a greater effect than uncommon ones. d. it is based on an isotope’s charge, so negatively charged isotopes have a greater effect than positive ones. select the best answer from the choices provided a b c d