名詞 a.:

1. a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation
   同義詞：angstrom, angstrom unit, A
2. any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes
   同義詞：vitamin A, antiophthalmic factor, axerophthol, A
3. one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
   同義詞：deoxyadenosine monophosphate, A
4. (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
   同義詞：adenine, A
5. the basic unit of electric current adopted under the Systeme International d'Unites
   同義詞：ampere, amp, A
6. the 1st letter of the Roman alphabet
   同義詞：A, letter a
7. the blood group whose red cells carry the A antigen
   同義詞：A, type A, group A