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 正确答案是D 请问，D中have been dated at。。。是什么意思，什么叫have been，感觉用了完成时态，有一直都在dated的感觉 |
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