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

--- 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→)A. who; ninthB. that; ninethC. /; ninethD. 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。

这个答案应该是对的