It is difficult to say precisely how many species of insects there

Question

It is difficult to say precisely how many species of insects there are on Earth, in part because it is often hard to distinguish between a separate species  and a variety of one, but scientists who have to give an estimate usually place  the number at around 950,000 unique species.

A. and a variety of one, but scientists who have to give an estimate usually place 
B. and a variety of them, with scientists having to give an estimate usually placing 
C. and a variety of one, but scientists who give an estimate usually placed 
D. or a variety of them, but scientists who have given an estimate usually placed 
E. or a variety of them, but scientists who have an estimate to give usually placing

Source- Prep4Gmat

shameekv1989 · Answer

I would go with C on this one.

It is difficult to say precisely how many species of insects there are on Earth, in part because it is often hard to distinguish between a separate species and a variety of one, but scientists who have to give an estimate usually place the number at around 950,000 unique species.

A. and a variety of one, but scientists who have to give an estimate usually place
B. and a variety of them, with scientists having to give an estimate usually placing
C. and a variety of one, but scientists who give an estimate usually placed
D. or a variety of them, but scientists who have given an estimate usually placed
E. or a variety of them, but scientists who have an estimate to give usually placing

we need between and contruction so D and E are out.

"them" in B looks ambiguous as it can be either (a separate species and variety of species of insects) or (separate species and variety of species within those in separate species)

Secondly scientists who have to sounds as if they are forced to give a number - I mean it sounds kinda awkward to say that way.

Therefore I would go with C.

DinoPen · Answer

I said A.
First part: You are comparing "between a separate species and " ... "a variety of one"
I took out the phrase "a variety of them" since "them" is plural and you are comparing species with itself. However, even to me this was shaky logic and moved on.

The deal breaker for me was the verb "place". You want present tense. At the very beginning of the sentence, the verb is present. (keeping parallel)

Abhi077 · Answer

OFFICIAL EXPLANATION

This sentence may sound weird, but let's try to understand what the sentence is trying to say: it's hard to count the number of species because you can't always tell whether a creature is part of a distinct species or if it's simply a variety or subcategory of a species, but scientists can give an estimate. 
In that case, choice (A) is better than (B); choice (B) uses "a variety of them," and "them" is plural, but really the variety refers to "an existing species," which is singular. Choices (D) and (E) have that problem, too, so we can eliminate them. 
Choice (C) mixes up a present tense and a past tense verb ("give" and "placed") in a way that makes no sense. Although the sentence may sound weird as it is, it's actually the best option.

The correct answer is Choice (A) .

It is difficult to say precisely how many species of insects there

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It is difficult to say precisely how many species of insects there

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615 to 715 in 15 Days (Ria's Strategies)

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