问题补充：
Peter is a boy. His younger brother is .
A. ten-years-old ; six years old
B. ten years old ; six years old
C. ten years old: .six-years-old
D. ten-year-old ; six years old
答案：
A. ten-years-old ; six years old
B. ten years old ; six years old
C. ten years old: .six-years-old
(答案→)D. ten-year-old ; six years old
解析：第一空ten-year-old是复合形容词，表示“十岁的”，三个词之间有连字符，名词用单数。第二空做表语，不用连字符，名词用复数。
