Sentence Correction

Scientist 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 milion years ago, pushing back by more than 150,000 years the earliest date /when it is known that humans make/ stone tools



when is is known that humans make
at which it is known that human had made
at which humans are known to have made
that humans are known to be makin
of humans who were known to make.

correc A. is C, please, explain, what wrong with A

thang wrote:Scientist 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 milion years ago, pushing back by more than 150,000 years the earliest date /when it is known that humans make/ stone tools



when is is known that humans make
at which it is known that human had made
at which humans are known to have made
that humans are known to be makin
of humans who were known to make.

correc A. is C, please, explain, what wrong with A

Hello, Thang.

A is obviously wrong, because humans MADE stone tools long time ago.

The fact that human made the tools is known to people in the present, so we have to use present tense like 'is known' or 'are known'. But they made the tools in the past. The correct answer should be Present + Past as is in C.

Sorry my friend

in A, it is "made" not "make". So why it is wrong"

this is question 41 of the test set 12 in "15 test set" on the internet.

pls, help

i found out today that humans used these stones 15,000 years ago

according to A,
it is today's date that gets shifted back

thank you barron, it seems that "when" in A is modifying "it is" and "at which" in C is modifying "to have made" ? but why "when" can not modify "that humans made"

Pls explain.best regards

thang wrote:Sorry my friend

in A, it is "made" not "make". So why it is wrong"

this is question 41 of the test set 12 in "15 test set" on the internet.

pls, help

What is the OA??