Pollution is a problem associated with urban sprawl, but not with upward growth.

A60-gram sample of radioactive isotope is measured after two half-lives have passed. what pair of percentages describe the sample after two-half lives? a.) 50% unchanged and 50% stable b.) 25% unchanged and 75% stable c.) 12.5% unchanged and 87.5% stable d.) 75% unchanged and 25% stable