Mary is girl,but she knows a lot about science.

问题描述:

Mary is girl,but she knows a lot about science.
A.a eight-year-old B.eight-year-old
C.eight years old D.an eight-year-old

D.an eight-year-old为什么?加上连字符,eight-year-old就相当于一个形容词再加上eight 的e是元音,所以前面用an