Down syndrome, the most common genetic condition in the United States, is also called trisomy 21. What does this mean?

(PLEASE ANWSER ONLY IF YOUR SURE )

A)

A person with Down syndrome has two copies of chromosome 21 but they're fused together. During early embryo development, these chromosomes didn't segregate properly.

B)

A person with Down syndrome has three copies of chromosome 21 instead of two copies. Two sperm cells simultaneously fertilized the egg cell.

C)

A person with Down syndrome has only one copy of chromosome 21 instead of two copies. Either the sperm cell or the egg cell was lacking this chromosome.

D)

A person with Down syndrome has three copies of chromosome 21 instead of two copies. An extra chromosome 21 was present in either the sperm or the egg cell.