请教OG 12 78的所有指代

OG12-78代词指代

78. 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
此题选D。
但是我一直有困惑：对于从句的that/which指代和代词的it/those指代，如果碰到了A of B的情况下到底是指代谁～
比如B 选项的，Fossils of the arm of a sloth, that .....中的that是指代fossils还是是sloth，还是都可能？
我由此还想到的 A of B blabla...., so it....那么it是指代A还是B，还是both are ok?
每次做题，碰到类似的总觉得心里发虚，谁知道能告诉一下，先谢谢啦。