A subrule of the CahnIngoldPrelog system specifies that higher mass number takes precedence over lower when distinguishing

Question:

A subrule of the Cahn–Ingold–Prelog system specifies that higher mass number takes precedence over lower when distinguishing between isotopes. (a) Determine the absolute configurations of the reactant and product in the biological oxidation of isotopically labeled ethane described in Section 7.2.