---John knows so much about the alien.Yes you know he is only eight-year-old boy.A. a

问题补充：

---John knows so much about the alien.

--- Yes, you know he is only eight-year-old boy.

A. a

B. an

C. the

D. /

答案：

A. a

(答案→)B. an

C. the

D. /

解析：第一次提到某物时，要用不定冠词，an或a，又因为eight的首个音标为元音音标，故选B