是a eight-year-old 还是an eight-year-old

是a eight-year-old 还是an eight-year-old __(a) eight-year-old is a （）girl.答案是13-year-old为什么eight-year-old不对 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 he is ____boy 1.a eight-year-old 2.an eight-year-old 3.an eight-years-old 4.a eight year old eight years old与eight- year-old She is a ______ girl .A .eight years old B.eight year old C.eight-years-old D.eight-year-old eight years和eight-year-old和eight-years-old和eight year oldhe is an____boy?A.eight years B.eight-year-old C.eight-years-old D.eight year old选哪个,我刚刚查了一下,有个人说eight-year-old在名词前,eight years old在be动词后.还 a girl of 后面加eight years old还是eight-year-old?为什么,麻烦详细一点在句子：Mary is a girl of_________中 He is __,and he has__sister.A.ten years old,an eight-year-old B.ten-year-old,eight year old 是a one-year-old boy还是an one-year-old boy an 【】-year-old boy括号里能填的单词都写出来,比如：an 【eight】-year-old boy注意是【an】不是【a】! she is an eight years old 还是she is an eight-year-old girl对要原因啊 eight -year old 与 eight years old的区别 a eleven-year-old还是an eleven-year-old he is ______boy A.an eight years old B .an eight-year-old两个好像都行啊,奥不懂啊 a eight-year-boy还是an eight-year-boy请高手解答, 是the old man has a five years old grandson还是the old man has a five-year-old grandson?