In previous studies in which features of lichen communities were used

Question

In previous studies in which features of lichen communities were used as indicators of pollution, the
communities examined were located on tree stems 1–2 meters above ground, though lichen communities
on lower branches have shown to be more sensitive to changes in the air environment.

A. have shown to be
B. have since been shown to be
C. show themselves to be
D. are shown to be
E. are shown as

OA

Show Spoiler

after some discussions

A. have shown to be
B. have since been shown to be
C. show themselves to be
D. are shown to be
E. are shown as

KarishmaB · Accepted Answer

Responding to a pm:

It all needs to make perfect logical sense.

- Previous studies examined lichen communities which were 1-2 meters above ground. 
- It has been shown that lichen communities on lower branches are more sensitive. (Later studies examined lichen communities on lower branches)

First note that you need to use passive. Lichen communities didn't themselves show. They 'have been shown'. The scientists did the 'showing'. Hence A and C don't work.

Use of present perfect is preferable instead of simple present because it has already been shown that lichen communities on lower branches are more sensitive. Probably later studies were conducted to figure it out. Since we already have the conclusion, it means the 'showing' is already done. Therefore, 'have been shown' is better than 'are shown'. Also notice the use of 'since' in option (B). It makes complete logical sense once you put the word 'since'. Previous studies examined lichen communities which were 1-2 meters above ground because at that time, it was not known that communities on lower branches are more sensitive. Since then, studies (later studies) have been conducted on lichen communities on lower branches and it has been found that they are more sensitive. Now it's not odd that previous studies used lichen communities that were higher up.

Vips0000 · Answer

More of logical reasoning question

Only ans B fits the contrast. Ans B it is.

Jp27 · Answer

IMO A.

D and E wrong because of active voice. we need passive.
"themselves in B makes no sense.
B is wrong because we have already used though in the sentence to show contrast so since is nt required
The last part reads ->
though lichen communities on lower branches   have shown to be   more sensitive to changes in the air environment, the 
communities examined were located on tree stems 1–2 meters above ground
so A fits the bill.

what's the OA?

daagh · Answer

The word previous studies indicate that there are going to be some later studies. While the communities happen to survive at 1 to 2 meters above ground ( which is the lower part of a tree), the later studies reveal that these communities do not survive at such low heights due to their sensitivity to changes in the air environment at those low levels. This is what the text means to say. The contrast reveled by the later studies are brought only in choice B, by using the time marker, sine.

Grammatically too, you do require a passive voice in the second part to be parallel to the passive voice in the first part. A and C are out for that reason. Among B, D and E, choice D and E miss the time-marker ‘since’ , thus leaving the question of when the contrast was established, wide open

gmatbull · Answer

@Daagh,
supposing option B were " have BEEN shown to be ", would it suffice?
In that case, B would be past perfect and passive too.

Meanwhile the correct response is  B 
Why must  "since"  be there?

Vips0000 · Answer

'Since' is the most important part of the option B. As mentioned in earlier post, only this option fits the contrast presented in original sentence, and since helps to create that fit.
As per your question, whether u can omit since and then will it make sense.. ans should be no. If you read the entire sentence without since, it makes no logical sense.

Arvars · Answer

Oh!!! I eliminated C,D and E and was left with A and B because of the tense form used. Here we need a perfect tense as it is a previous study that is mentioned in the question.

Was not sure why should A be eliminated, although B sounds weird but the usage of 'since' is important to introduce the a contradiction and so the correct answer should be B

Annet1524 · Answer

In previous studies in which features of lichen communities were used as indicators of pollution, the
communities examined were located on tree stems 1–2 meters above ground, though lichen communities
on lower branches have shown to be more sensitive to changes in the air environment.

A. have shown to be
B. have since been shown to be
C. show themselves to be
D. are shown to be
E. are shown as

OA

Show Spoiler

after some discussions

A. have shown to be -  active voice - wrong 
B. have since been shown to be -  passive voice - correct  
C. show  themselves  to be they con't show themselves
D. are shown to be - strange constraction since it is not clear shown by whom
E. are shown  as  - wrong meaning

VerbalBot · Answer

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In previous studies in which features of lichen communities were used

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In previous studies in which features of lichen communities were used

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