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

问题补充：

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

--- Yes. He is Peter’s 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：第一空应填引导定语从句的引导词，同时引导词在从句中作主语，指代先行词the boy，故排除答案C、D；第二空是说他的第九个生日，该用序数词，nine的序数词是ninth，故选A。