Gallium Has an Average Atomic Mass of 69 ${ }_{31}^{72} \mathrm{Ga}$

Gallium has an average atomic mass of 69.7 amu.In a typical sample,60.4% of Ga exists as Ga-69 (68.9257 amu) .What is the identity and the atomic mass of the other isotope?

A) ${ }_{31}^{72} \mathrm{Ga}$ ;70.9 amu
B) ${ }_{31}^{70} \mathrm{Ga}$ ;70.9 amu
C) ${ }_{31}^{71} \mathrm{Ga}$ ;70.9 amu
D) ${ }_{31}^{71} \mathrm{Ga}$ ;71.9 amu
E) ${ }_{31}^{72} \mathrm{Ga}$ ;71.9 amu