my sister could swim__________the age of ten A on B of C at D in

my sister could swim__________the age of ten A on B of C at D in

C
at the age of ten = When she was ten years old.10岁时
不明白的再问哟,请及时采纳,多谢!

c
at the age of

c 在什么的年纪

选C age表年龄的词就是和at 连用的

选C at
at the age of……在（多大）的时候