定语从句---Do you know the boy ___ is sitting next to peter.---Y

定语从句
---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
我选的B,我觉得没错啊!答案给的是A,我觉得两个都对.
Do you know the girl_____?A.he often talks to B.to who he often talks C.to that often talks D.whom he often talks to
答案给的是A,我觉得D也没有错啊,可以把关系代词写出来啊,而且这里也是做宾语啊!