Scientists have dated sharp-edged flakes of stone found in the fine-grained sediments of a dry riverbed in the Afar region of Ethiopia to between 2.52 and 2.60 million years ago, pushing back by more than 150,000 years the earliest date when it is known that humans made stone tools.
- when it is known that humans made
- at which it is known that humans had made
- at which humans are known to have made
- that humans are known to be making
- of humans who were known to make
答案选C，但是我不知道B哪里不对呢？为什么此处不可以用形式主语。
还有一点就是为什么要用 are known? 如果已经通过新的发现证明其不是一个客观事实，是不是应该用were更合适呢？
关于从句时态方面，我一直有困惑，请看下面一题 白勇P224 - 34
His studies of ice-polished rocks in his Alpine home-land, far outside the range of present-day glaciers, led Louis in 1837 to propose the concept of an age in which great ice sheets had existed in now currently temperate areas
A . in which great ice sheets had existed in now currently temperate areas
B. in which great ice sheets existed in what are now temperate areas
C. when great ice sheets existed where there were areas now temperate
D. when great ice sheets had existed in current temperate areas
E. when great ice sheets existed in area now that are temperate
正确答案是B
但是我不太理解为什么B中从句的existed不用过去完成时态，毕竟概念是1837年提出来的，应该和这个年代形成对比，有些糊涂了，希望有人能告诉我这两题我犯了什么错误，还有从句中的时态有没有特别要注意的地方呢？？先谢过大家了！
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