1.the blood group whose red cells carry the A antigen
2.a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation
3.any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes
4.one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
5.(biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
6.the basic unit of electric current adopted under the Systeme International d'Unites;
a typical household circuit carries 15 to 50 amps
7.the 1st letter of the Roman alphabet
a,拉丁文、英文等語言中字母的第一個,在英文中也是一個不定冠詞。這個字母在很多領域均有應用,代表的含義各不相同,比如物理中表示電流單位,遺傳學中表示腺嘌呤,游戲中表示攻擊,網絡語言中表示詐騙等等。