a是虚词还是实词

实词。

art. 一(个)每一(个)任一(个)

n. 英语字母表的第一个字母学业成绩得优者表示“最好”的符号

例子

a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron) used to specify wavelengths of electromagnetic radiation

any of several fat-soluble vitamins essential for normal vision prevents night blindness or inflammation or dryness of the eyes

one of the four nucleotides used in building DNA all four nucleotides have a common phosphate group and a sugar (ribose)

(biochemistry) purine base found in DNA and RNA pairs with thymine in DNA and with uracil in RNA