Which of the following describes how the weighted average atomic mass of an element having two isotopes is determined? A. (mass of isotope A + mass of isotope B)/2 B. (mass of isotope A + mas of isotope B) % abundance of isotope A+\% abundance of isotope B) C. (mass of isotope Aabundance of isotope A) + (mass of isotope B) % abundance of isotope B) D. 2(mass of isotope A + ma mass of isotope B) + (% abundance of isotope A + % abundance of isotope ope B)/2