题目选项分析

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.

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.