GWD-8-34

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

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

b,c,e的错误我都知道在哪里了，对于a我想问一下问题是处在and前面的逗号吗？有人说是made it的it指代有问题，但是我觉得这边它指代a sloth没有什么问题呀？请牛牛们指点，还有一个就是d选项的have been dated 可以这样用吗？

顶一下，防止被淹没

多谢版主