Down Syndrome Could Not Be Treated by Gene Therapy Because

Down syndrome could not be treated by gene therapy because

A) scientists do not know which faulty genes cause the syndrome.
B) Down syndrome is caused by possessing extra copies of genes of an entire chromosome, and adding more genes would not help.
C) liposomes and nasal sprays cannot be used due to abnormalities in the respiratory tract of Down syndrome individuals.
D) there are too many genes involved in the syndrome.
E) the gene therapy treatment could not be performed on an adult individual.