Scientists are investigating the possibility of microbial life in Lake

This is a tricky one.

But first, let's see the basics:

You should use THAT when you have to define or provide important information about a noun. Without that information you cannot identify the noun. For example:
I want to apply to a business school THAT ranks as one of the best schools in the world.

But if you can identify the noun, THAT is not required. In that case, you could use ", which". For example:

I want to apply to Harvard Business School, WHICH ranks as one of the best schools in the world.

You can see that "ranks....world" is just additional information. If you read only "I want to apply to Harvard Business School", you can identify the school. More information is not required.

Now let's see the question:

If we only read "the lake's ecosystem is one", we can see that it would be wrong. "ONE" has to be defined; we need THAT.

But, why can option E be correct if it doesn't have THAT? And here comes the trick:

We can ommit THAT when the object of the verb of the clause that defines (in this case, "they believe...") is the noun that it is being defined.

That's why B could also be correct, but it is wordy, and E is concise. So, E is better and correct.

I think you should read the MGMAT SC book if you are begining to study SC. It's big, but it is good!

PS. Please, if someone disagrees, please reply.

The ecosystem is one that has no sense; \(that\) is also implicit in this context

C IT refers to eco or one I have doubt......move on

D that is ia tense weird here. I believe TO BE ill

E is the best

Under the circumstances, A looks like the best answer. "they believe to be very similar to" is not a complete clause. One..that construction looks a must here and with that option B is simply wrong because of repetitive pronount it.

I think that in A is parallel to that later.. E seem to disrupt that connection .. Also the position of two to's so hear causes some confusion.. Please provide the idea why A is wrong ?

Posted from GMAT ToolKit

I somehow have always associated "believe" with the phrases "believed to be" or "believe it to be" - "This criminal is believed to be the most dangerous criminal in the country..." or such.

I just feel it is the "most appropriate" choice of words. I do not think A is wrong, but I chose option E to be the best one available.
Hope it helps!

I think the correct answer is "E" because the idiom is "believe X to be Y"

The issue here is the phrase "that they believe is." The proper idiomatic form is they believe to be. Eliminate (A).

A vertical scan reveals a 2-3 split between "that they" and "they." While "that" is not incorrect per se, here it is unnecessary because the pronoun "one" sufficiently introduces the modifier. Thus, eliminate (B) as well.

(C) contains the word "it," which is redundant because of the word "one" that appears previously. (C) also does not use the idiomatic believe ... to be construction. Eliminate.

(D) is both unidiomatic and redundant, using "is" as the verb and the pronoun "that." Eliminate.

Therefore, (E) is correct. This choice uses the correct idiom, "believe to be," and doesn't insert any unnecessary pronouns.

Note that this sentence is not necessarily the most elegant or efficient way to communicate the intended meaning. Do not let this bother you. Focus on the specific issue being tested. The underlined portion is your best clue as to what's being tested, though the non-underlined portion usually has an impact on the issue being tested as well. On Test Day, you may see some Sentence Correction questions for which you could construct a better sentence than any offered, but thinking that way will not increase your GMAT score. Always focus on choosing the correct option on the screen in front of you.

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