T-4-Q1 天山-7-41 again

echo19872010

T-4-Q1 天山-7-41 again
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.

• A    when it is known that humans made
  
• B     at which it is known that humans had made
  
• C     at which humans are known to have made
  
• D     that humans are known to be making
  
• E     of humans who were known to make
  

查了之前的帖子，还是疑惑！
1.date 用when 或者 at which修饰都可
2. 关于时态，还是得建立在理解整句话的意思上，科学家确认尖石的年代是。。。，把人类最早使用石具的记录向前推了150000多年。（pushing back句是个小倒装，应为:pushing back the earlist date at which ...by more than 150000 years）
因此at which humans are known用现在时是对的，表示目前所有的记载，制造石器比are known要早，所以现在完成时和过去时都可以咯
那为什么A 不对？？？

piggycandys

at which 比 when更准确

xiruihuang

給你一個規律
time when。。。
In 2012（確切年份） when。。。
age in which
date at which
一般就這麼考

brunhildS

C的have made 求解！！！