Lhitachii

根本不同是 A 选项，to be caused by human beings in burning fossil fuels, 直接原因是 human beings。C 选项，to be caused by human beings' burning of fossil fuels。直接原因是 burning 。

Lhitachii

细读一下句子，全球变暖当然不是人本身的问题，而是人类使用燃烧燃料造成的，

Lhitachii

a phenomenon most scientists agree is caused by human beings' burning of fossil fuels 这句话实际是省略了that, 完整的为a phenomenon that most scientists agree is caused by human beings' burning of fossil fuels. 其中most scientists agree为从句的插入语。还有逻辑上说，人类本身不是原因，而人类的活动才是原因，希望能明白这点。