-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
网友回答
(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。