[求助]再请教一道语法题

31. GWD17-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.

A.      by which

B.       for which

C.      under which

D.      such that

E.       wherein

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参考答案是C，请高手指点原因，其他不对的原因是什么？谢谢

只有under an agreement是符合语言习惯的.

谢谢大牛，明白了