-Do you know the boy is sitting next to Peter?-Yes. He is Peters friend. They are celebra

问题补充：

-Do you know the boy is sitting next to Peter?

-Yes. He is Peters friend. They are celebrating his birthday.

A. who; ninth

B. that; nineth

C. /; nineth

D. which; ninth

答案：

(答案→)A. who; ninth

B. that; nineth

C. /; nineth

D. which; ninth

解析：who 引导定语从句，在从句中作主语，其他的词为表示人的名词或代词，表示第几个生日要用序数词。