题目选项分析

Fossils of the arm of a sloth, that they found in Puerto Rico in 1991, has been dated at 34 million years old, thus making it the earliest mammal known onthe Greater Antilles Islands.

Fossils of the arm of a sloth, found in Puerto Rico in 1991, have been dated at 34 million years old, making the sloth the earliest known mammal onthe Greater Antilles Islands.