One half-life of a radioactive isotope has passed when: A. all the parent nuclei have become daughter nuclei.
B. the number of daughter nuclei is twice the number of parent
nuclei.
C. the number of daughter nuclei is half the number of parent nuclei.
D. the numbers of parent and daughter nuclei are equal.

answer: 45

explanationyou have to add as well to be it could be

the cell is the smallest structural and functional unit of living organisms, which can exist on its own. therefore, it is sometimes called the building block of life. some organisms, such as bacteria or yeast, are unicellular—consisting only of a single cell—while others, for instance, mammalians, are multicellular.