How old is her sister

解题思路： 见解答过程
解题过程：
How old is her sister?
A.It's ten years old. B.She's ten years old.
C.She's ten years' old. D.She is ten year's old.
答案：B
解析：首先根据问句可知问的是 her sister，故答语应该使用主语she,years old是“....岁”的意思。故选B .
最终答案：略