一道高中英语题，求详解。。希望大家可以帮帮我~

Question

One of the most important issue they had to consider in the UN climate conference in Copenhagen was ____ rich-poor cooperation on climate change.
A.one B.this C.that D.which 答案是C，怎么解释啊？
rich-poor cooperation on climate change是合作关系变化，还是气候变化啊？

Answer 1

One of the most important issue they had to consider in the UN climate conference in Copenhagen was__that__ rich-poor cooperation on climate change.
主语是One of the most important issue
谓语是was
宾语是that, that的具体内容是 rich-poor cooperation on climate change.
they had to consider in the UN climate conference in Copenhagen 修饰的是主语issue
rich-poor cooperation on climate change是贫富国家之间关于气候变化的合作.

Answer 2

rich-poor cooperation on climate change 发达国家与贫困国家间关于气候变化的合作
that引导的是同位语从句，其实就是ONE OF THE MOST IMPORTANT ISSUE WAS THAT ......
后面的they had to consider 是定语从句，省略了which 或 that ，做宾语可以省略。
希望对你有帮助。