The agreement, the first to formally require industrialized countries to cut emissions of gases linked to global warming, is a formal protocol by which 38 industrialized countries must reduce emissions of these gases by 2012 or face heavy penalties.

1. by which

2. for which

3. under which

4. such that

5. wherein

答案是C，为什么？

谢谢解答！

挑着做的，呵呵

前面的还没做呢

也有by protocol地用法的~~为什么C阿？？