--I hear that Toms sister is a teacher here.--Yes the girl is standing under the tree is

问题补充：

--I hear that Toms sister is a teacher here.

--Yes, the girl is standing under the tree is his sister.

A. who B. whom C. whose

答案：

解析：句意：我听说汤姆的姐姐是这儿的一个老师。——是的，站在那边树下的女孩是他的姐姐。下文先行词the girl为下文定语从句的逻辑主语，指人，故引导词用who，选A。