I’ve always had a soft spot for this question, just because it has such a wide variety of issues and errors. It also has an interesting little verb tense thing that really doesn’t matter at all: you could easily fixate on the difference between "was dated" (past tense) and "has/have been dated" (present perfect tense), but you don't have to choose between them at all. There are plenty of other issues to worry about instead.
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(A) sloth found in Puerto Rico in 1991, and dated at 34 million years old, made it the earliest known mammal of
The “it” (which presumably refers back to the sloth) is the first thing that jumps out at me. “Fossils of the arm of a sloth… made [the sloth] the earliest known mammal…” Huh? That makes no sense. How the heck would the fossils of a sloth make the sloth the earliest known mammal of the Greater Antilles?
The much smaller issue is that I see no real need for that first comma, but that’s unlikely to ever be a deciding factor on a GMAT question. But the nonsensical main clause is enough to wipe out (A).
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(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
This time, the pronoun “they” is the first thing that catches my eye. Who the heck is “they”? The fossils?
Plus, we have a subject-verb problem: “fossils… has been dated” is spectacularly wrong. (B) is gone.
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(C) sloth that was found in Puerto Rico in 1991, was dated at 34 million years old, making this the earliest known mammal of
Subject-verb fail again: “fossils… was dated.” Nope.
But there’s more: the phrase “that was found in Puerto Rico in 1991” seems to modify the sloth itself, and that makes no sense. Unless you think that the sloth had actually been alive for 34 million years. Maybe moving really slowly helps you live longer?
Finally, the word “this” is used as a singular pronoun, and that’s an unacceptable use of the word “this” on the GMAT. I’ve only ever seen “this” used as an article on the GMAT (“this week” or “this magnificent GMAT forum”), but never as a pronoun.
So we have lots of good reasons to eliminate (C).
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(D) sloth, found in Puerto Rico in 1991, have been dated at 34 million years old, making the sloth the earliest known mammal on
This one looks pretty good. The subject-verb agreement works: “fossils… have been dated at 34 million years old…” Cool. The modifier “found in Puerto Rico in 1991” is nicely set apart with commas, so it seems to refer to the entire phrase “fossils of the arm of a sloth”, and not just the sloth itself, as we saw in (C).
And the last part of the underlined portion (“making the sloth the earliest known mammal…”) actually names the sloth instead of using a pronoun, and that adds a bit of extra clarity. It might not be strictly necessary, but it’s nice. We can keep (D).
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(E) sloth which, found in Puerto Rico in 1991, was dated at 34 million years old, made the sloth the earliest known mammal of
Thanks to the modifier “which”, which typically modifies the immediately preceding noun, it really sounds like the sloth itself was found in Puerto Rico in 1991, and that makes no sense. (I also can’t make much sense of the comma placement in that part of the sentence, but again, comma placement isn’t really a major emphasis on GMAT SC.)
We also have the same problem that we had in (A): “fossils… made the sloth the earliest known mammal…” That’s nonsense, too.
So we can ditch (E), and (D) is our answer.
Dating of the fossils is not an ongoing process + it is a thing of past else it wont make sloth earlies known mammal, can you please help me understand taking into consideration this as to why the usage of "have been" correct
- The “it” (which presumably refers back to the sloth) is the first thing that jumps out at me. “Fossils of the arm of a sloth… made [the sloth] the earliest known mammal…” Huh? That makes no sense.
My view : It is just saying that particular evidence made the sloth the earliest know mammal? What is wrong with the usage of "it" referring to sloth? How else to put this?