PREP 135,有人问过，但没有得出肯定的回答

有人说是插入语，也有人说是这种情况的主语从句中that可以省略，至今没有定论，盼大家讨论！！！

曾经有个时候，美国大栗子树是如此的流行，以致于，据说一只松鼠可以从一棵树跳到另一棵树，脚步落地地从纽约州到G。（就是说纽约到G的路上都种了这个树）

it be said that 是个习惯的用法。that 能不能我存疑，

不过B， a squirrel was said that 好像挺古怪的，肯定不对的。语法上似乎没有问题，翻译起来有点不通：大栗子树是如此的流行，以致于，一只松鼠被说成可以……，

T-4-Q1 TS-7-41

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

答案就是C，用的就是humans are known

不是说问题的关键在一个是人，一个是动物吧？

或者说sb be known/said之类的只可以加to do 吧？

请指教！

给个例子

A recent poll indicates that many people in the United States hold a combination of conservative and liberal political views; i.e., they denounce big government, saying government is doing too much and has become too powerful, while at the same time supporting many specific government programs for health care, education, and the environment.

新prep214