Down Syndrome Results from Trisomy of Chromosome 21

Down syndrome results from trisomy of chromosome 21.Which of the following is NOT an example of how this may have happened?

A) translocation of chromosome 21
B) failure of the homologous pair for chromosome 21 to separate in meiosis I
C) inversion of a portion of chromosome 21 during S phase
D) failure of the sister chromatids in a duplicated chromosome 21 to separate in meiosis II