An average sample of lithium contains 7.5% lithium-6 and 92.5% lithium-7. Lithium-6 has an atomic mass of 6.015 amu and lithium-7 has an atomic mass of 7.016 amu. What is the average atomic mass of a sample of lithium? A B C D 6.09 amu 6.49 amu 6.52 amu 6.94 amu​