The narrator in "The Raven" tells himself that the tapping
he heard was
A. a "visitor entreating entrance."
B. "the wind and nothing more."
C. both a. and b.

About 95% of the time down syndrome is caused by “trisomy 21.”the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.