题目选项分析

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