标题: 求助新prep1中的256 [打印本页] 作者: 小禽小兽 时间: 2010-8-25 10:32 标题: 求助新prep1中的256 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 答案是C. 不明白的是为什么句子中要用完成时态，请各路NN们帮俺解答作者: 小禽小兽 时间: 2010-8-27 02:11
这个也顶起来作者: jasonhaha 时间: 2010-8-27 06:16
看看主句用的是什么事态呀：）作者: gesteven 时间: 2011-2-19 07:42
是哦，一语点中