大神解决下我这个小小的问题。。在CD上没搜到我想要的解释。。

lisney

Fossils of the arm of a sloth found in Puerto Rico in 1991, and dated at 34 million years old, made it the earliest known mammal of the Greater Antilles islands.

A.       sloth found in Puerto Rico in 1991, and dated at 34 million years old, made it the earliest known mammal of

B.       sloth, that they found in Puerto Rico in 1991, has been dated at 34 million years old, thus making it the earliest mammal known on

C.       sloth that was found in Puerto Rico in 1991, was dated at 34 million years old, making this the earliest known mammal of

D.       sloth, found in Puerto Rico in 1991, have been dated at 34 million years old, making the sloth the earliest known mammal on          (D)

E.        sloth which, found in Puerto Rico in 1991, was dated at 34 million years old, made the sloth the earliest known mammal of

其他各种it,this 不能指代导致出错我都能看出来然后排除来选择，但是我就是不明白为什么要用 have been dated at 时态？？搜CD上的帖子发现没人有这个疑问，，，呃，，这个怎么理解？难道是一直被鉴定为34 m old？年份鉴定不是应该是过去式么。。。？说不通啊我觉得。。。希望有大神解决下我这个小小的问题。。。。谢谢！！

lexuefeifelix

这里是被动，不是have been doing的形式，所以不是一直，用完成时虽然没有什么标志，但是根据句意可以理解为前面这件事对后面那件事有影响，所以用现在完成时是没有错的

sisichen666

其实就是句意理解的问题：在1991年被发现的fossils，到目前为止已经34million years......——一般被鉴定是XXX年都是以现在为基准而作出的结论，因此用have been dated

enkyklios

http://www.manhattangmat.com/forums/dated-at-vs-dated-to-be-t604.html  有个题挺类似