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 was found in Puerto Rico in 1991, was dated at 34 million years old, making this the earliest known mammal ofthe Greater Antilles Islands.