我查了下爱词霸, be known as 以...知名; 被认为是; 称为    be known to 为...所知

但是新东方蓝皮书上有一题是这样的:

Until a few centuries ago, any large bones discovered in the fields or caves of Europe, now known tobe large prehistoric animals, were usually assumed to be the remains of giants and were often.........

答案是 C bones now known to be those of large prehistoric animals, were usually assumed to be

为什么这里用known to be 呢?    known as 和 known to be 分别是什么意思呢...

BE KNOWN TO + 動作 人盡皆知
ex: He is known to be an excellent athlete.
BE KNOWN AS + 身份
ex: He has been known to us as a story writer.

BE KNOWN FOR + N以某事物知名

谢谢楼上..不过我还是不太懂..

to be an excellent athlete
as a story writer

那第１个不也是身份吗？　　你to be 后面加的也不是动作啊．．．