the taller of the two girls.为什么不用不是the taller in the two girls

of 表示同等关系,in 表从属关系
这以结构the taller in the two girls其实是the taller (girl) of the two girls 的省略式.都是girl ,故用of
再如：
【Of】 all the cities in China,Shanghai is the biggest.
比较：
Shanghai is the biggest city【 in】 China.上海和中国是从属关系,故用in