Li Lida a----boy crossed the Qiongzhou Channel in June 2000.

答案选a
解析：“基数词-year-old” 一般用来做定语修饰名词.
例如： 一个13岁的男孩：a thirteen-year-old boy
She was an eight-year-old girl that year.
而基数词+years old,常放在be动词后作表语.Anna is only five years old.