hard SC question. Please,explain to me, friends.

thang

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

celeste.kr

thang · Posted: Sat Apr 26, 2008 8:14 pm Post subject: help, pls,

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

barron · Posted: Thu May 08, 2008 7:40 am Post subject:

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

thang

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