As a geologist, you’re assessing the feasibility of determining the ages of Earth’s earliest rocks using radioactive dating. You estimate the number of half-lives that have passed for three different isotopes during Earth’s 4.5-billion-year lifetime, and from that you determine the number of atoms remaining today from 10⁶ atoms present at Earth’s formation. The isotopes you consider are carbon-14, uranium-238, and potassium-40. What are your estimates, and which isotopes do you conclude are suitable for radioactive dating?