XDF蓝皮中册P119T14???

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

新东方蓝皮上给出的是E.但是我查过些资料,更多人倾向C. 一个美国人解释说:

Scientists have dated sharp-edged flakes ... to between 2.52 and 2.60 million years ago, pushing back ... the earliest date at which humans are known to have made stone tools.

if you insert (E) "of humans who were known to make," the sentence makes the nonsensical statement of "date of humans" rather than date when humans made stone tools.

if you insert (A) "when it is known that humans made," the pronoun "it" has no logical referent.

那么为什么答案是E呢? C 或 A 哪里不好呢? 我倾向选A

这题我选C，顶一下，高手指教！

我觉得是C啊，A中when和that是不是有点wordy之嫌？