Q31.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.

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1. by which
2. for which
3. under which
4. such that
5. wherein

这题选C,是不是用了什么习惯用语啊??

求教