Nondisjunction is the failure of homologous chromosomes to separate during meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. As a result, both homologous chromosomes or both sister chromatids migrate to the same pole of the cell. This produces daughter cells with an imbalance of chromosomes. A cell biologist examines the final products of meiosis in an earthworm (2n=36) and finds 2 cells with 20 chromosomes, and 2 cells with 16 chromosomes. Most likely this was because

a. 2 pairs of sister chromatids failed to separate during meiosis II
b. 1 pair of sister chromatids failed to separate during meiosis II
c. 2 pairs of homologous chromosomesfailed to separate during meiosis I
d. 1 pair of homologous chromosomes failed to separate during meiosis I