A 3-Year-Old African-American Child Is Diagnosed with Sickle Cell Anemia

A 3-year-old African-American child is diagnosed with sickle cell anemia.The parents know that sickle cell anemia is hereditary but do not understand why their child has the disease,because neither of them has it.The nurse explains that:

A) At least one of the parents has to have the disease.
B) Only one parent has to have the disease or the trait.
C) Someone in previous generations had the disease.
D) Both parents were carriers of the sickle cell trait.