Lithium has two stable isotopes: Li-6 (mass = 6.015 amu) and Li-7 (mass = 7.016 amu). Which isotope is more abundant and why?
A) Li-6; because it has a lower mass
B) Li-6; because it has a higher mass
C) Li-7; because it has a lower mass
D) Li-7; because it has a higher mass