[求助]争议题啊？

manliangiggs

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

这道题蓝皮书给的答案是E,但我怎么看到还有C呢，到底是什么啊？盼指点。

Zeldaw

LZ看看这个吧，介绍了题目来源和出处,包括所有蓝皮书的题目。手打题目太费时间了。

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senecabbs

个人认为C是答案，E。。。反正我是不会选

Breath

I chose C