熟练劳动力所占的比重、技术经济特点、工会化程度、地理位置等是造成（）之间出现工资差别的主要原因。

A.不同年龄劳动力

B.男性和女性劳动力

C.不同民族劳动力

D.不同产业

-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