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(First of all,this is grammar question.)

  • İn the 1970s scientists found out that chemicals_____ into the atmosphere______the ozone layer.

A)having been released /destroyed (which is picked)

B)released /had been destroying (which is correct )

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Option A would read as follows (with a few commas added for clarity):

In the 1970s scientists found out that chemicals, having been released into the atmosphere, destroyed the ozone layer.

If you include these commas, the sentence is grammatical. However, it states that the ozone layer was completely destroyed, which is not the case. It also suggests that "chemicals" in general leaked by chance into the ozone layer, when in actuality the author has a few specific chemicals in mind. (Presumably, there is an accompanying reading that clarifies this.)

As a pure grammar question, you can rule out option A because commas are required around such "gerund appositives." This is a bit subtle, however, so I think it's more likely that the author intended this as a reading-comprehension question instead, as above.

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Both answers are **grammatically* correct. However, there's a difference in meaning.

İn the 1970s scientists found out that chemicals, having been released into the atmosphere, destroyed the ozone layer.

The above sentence (with the 2 commas added) means "the chemicals" just "destroyed the ozone layer" at one point (at a moment in time or just in a day).

İn the 1970s scientists found out that chemicals released into the atmosphere had been destroying the ozone layer.

The above sentence means "the chemicals that had been released into the atmosphere" had been "destroying the ozone layer" over a period of time (a process over many months/years).

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  • "In the 1970s scientists found out that chemicals_____ into the atmosphere______the ozone layer."

The option with the "released/had been destroying" is indeed grammatically correct.

The sentence reads as follows:

  • "In the 1970s scientists found out that chemicals that were released into the atmosphere had been destroying the ozone layer."

So because of the chemicals the ozone layer was being under the process of destruction and may have been possibly destroyed by the time the scientists figured it out.

Still, since this question concerns only grammar the option that says it is correct is indeed correct.

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    I must disagree that had been destroying " implies that the destruction was complete by 1970, or by any date.A simple past "destroyed" would suggest that the destruction was complete. – David Siegel Jun 7 at 19:02

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