求解释GWD-8-Q34

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.

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

B.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

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

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

E.sloth which, found in Puerto Rico in 1991, was dated at 34 million years old, made the sloth the earliest known mammal of
这道题选D          A错在哪了？   如果把it改成the sloth的话A对吗？？             大家帮帮我呀~~~~~~~~~~~