[求助]補充题 Q52--->求教(再問)

52. 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 it is known that humans have made
D that humans are known to be making          A

E  of humans who were known to make

請問一 下A或C哪個好?

C用过去时是错的.

謝謝lilyzy...可我剛剛打錯了,,B跟C打成一樣的,,

如果是現在這樣 ,,那個好呢?

完成时不通. made tool是过去发生的, 所以要用过去时.