The common ancestors of Australian land- and tree-dwelling kangaroos had prehensile (grasping) tails and long opposable thumbs, attributes that are well adapted
to tree-dwelling but offer kangaroos few advantages on land. It is hardly surprising, therefore, that land-dwelling kangaroos eventually lost these attributes; what is puzzling is the fact that all modern tree-dwelling kangaroos now lack them as well.

Which one of the following, if true, most helps explain the puzzling fact cited above?

(A) Modern tree-dwelling kangaroos must back down tree trunks slowly and carefully, but the common ancestors of modern tree-and land-dwelling kangaroos used their opposable thumbs to descend trees quickly headfirst.

(B) Modern tree-dwelling kangaroos are smaller than most modern land-dwelling kangaroos but larger than their common ancestors.

(C) Modern tree-dwelling kangaroos' tails cannot grasp branches, but they are somewhat longer and more flexible than those of modern landdwelling kangaroos.

(D) Modern tree-dwelling kangaroos are descended from species of land-dwelling kangaroos that had been land-dwellers for many generations before modern tree-dwelling kangaroos started to develop.

(E) Modern tree-dwelling kangaroos have smaller and weaker hind legs than modern landdwelling kangaroos, and they move more slowly on land than do modern land-dwelling kangaroos.

The OA is D.

Why D is the option which most helps explain the puzzling fact?