what are "the big three"?

contaminated drinking water,poor sanitation, and air pollution.

那棵大树是什么？？？

好像有一种解释是当今世界面对的三大问题
好像是人口,空气污染还有什么的,不记得了..

...楼上的,是three,不是tree

the Big Three(美国三大汽车公司）

The Big 3 refers to the top three auto makers: Ford, Chrysler and General Motors, all of whom make their home and headquarters in Detroit.

应该是“三巨头”吧。所指不一，二战中指罗斯福、丘吉尔和斯大林。