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. Please select the best answer from the choices provided A B C D