GWD-21-34 求教

AlphaBeta

题的争论焦点在A和C,到底选哪个呢,NN?

Q34:

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.

1. when it is known that humans made 
2. at which it is known that humans had made
3. at which humans are known to have made
4. that humans are known to be making
5. of humans who were known to make

AlphaBeta

不好意思,能不能劳驾NN再给看一看,刚才在逻辑题干中找到一个和这个语法题中时间表述相同的表达,如果这个用法成立,那么是不是说明C的致命弱点就是时态了呢?

GWD-9-Q40:

Agricultural societies cannot exist without staple crops. Several food plants, such as kola and okra, are known to have been domesticated in western Africa, but they are all supplemental, not staple, foods. All the recorded staple crops grown in western Africa were introduced from elsewhere, beginning, at some unknown date, with rice and yams. Therefore, discovering when rice and yams were introduced into western Africa would establish the earliest date at which agricultural societies could have arisen there.

本题:

....more than 150,000 years the earliest date....

C    at which humans are known to have made