- 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：句意：---你知道正挨着Peter 坐的那个男孩吗?---是的，他是Peter的朋友，他们正在庆祝他的第九个生日。定语从句中先行词是人用who, that都可以，因为关系词在句中作主语所以不能省略，某人的第几个生日用序数词，nine的序数词为ninth故选A。