An amniocentesis is a medical procedure that is used to detect many genetic disorders in humans before birth. The karyotype below shows the number and
structure of homologous pairs of chromosomes in a body cell of a human fetus taken
from this medical procedure.
What caused the chromosomal alteration in number 21?

A) a part of a chromosome was lost (deletion)
B) a duplicated chromosome failed to separate (nondisjunction)
C) part of the chromosome attached to another chromosome (translocation)
D) some of the genes on a chromosome were reversed (inversion)