How is the half-life of a radioactive isotope determined?
A. By measuring the time it takes for half the daughter nuclei in a sample to become parent nuclei
B. By measuring the time it takes for all the parent in a sample to become daughter nuclei
C. By measuring the time it takes for all the daughter nuclei in a sample to become parent nuclei
D. By measuring the time it takes for half the parent nuclei in a sample to become daughter nuclei