O.

名词 o.:

1. a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust
   同义词：oxygen, O, atomic number 8
2. the 15th letter of the Roman alphabet
   同义词：O, letter o
3. the blood group whose red cells carry neither the A nor B antigens
   同义词：O, type O, group O