Bill is an boy.A. eight years oldB. eight-years-oldC. eight-year-oldD. eight-year-olds

问题补充：

Bill is an boy.

A. eight years old

B. eight-years-old

C. eight-year-old

D. eight-year-olds

答案：

A. eight years old

B. eight-years-old

(答案→)C. eight-year-old

D. eight-year-olds

解析：表示多少岁数修饰名词，可用两字符将每个单词连在一起，不可使用复数形式，相当于一个形容词，故本题选C。