Calculate the reduced masses of (a) an HD molecule, (b) a $^{12} \mathrm{C}-\mathrm{H}$ unit and (c) a $^{13} \mathrm{C}^{16} \mathrm{O}$ molecule. Exact masses: $^{1} \mathbf{H}, 1.01 ;^{2} \mathrm{H}, 2.01 ;^{12} \mathrm{C}$
$12.00 ;^{13} \mathrm{C}, 13.00 ;^{16} \mathrm{O}, 15.99$.