He started walking when he was .A. 2-year-old B. 2 year old C. 2 years old

问题补充：

He started walking when he was .

A. 2-year-old B. 2 year old C. 2 years old

答案：

解析：句意：他在两岁时开始走。2-year-old两岁的，是形容词，一般作定语；2 years old两岁，是一个短语，常作表语。所以选C。