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.

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

2. 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

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

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

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

答案是C。但是，明显主谓不一致阿。正确答案为什么不是D呢？

题目意思是

a sloth(单数) 的 fossils(复数)的发现, 让这只sloth成为最早的哺乳动物

化石不能成为哺乳动物

我也做错了这道题

之所以用this, 就是要让句子意思更加明确-------this就是指代前面提到的那只sloth

如果把后面变成 the earlist fossils of mammal on the earth, 那就D最爽了