a question about structure of sentences

fire fly

Senior Member
vietnamese
Dear great people!
Could you please help me with this sentence?
Carbon dioxide may be absorbed by trees or water bodies, or it may stay in the atmosphere when ________, while it is only in the atmosphere that chlorofluorocarbons find their home.
A. by releasing emissions from cars B. released from car emissions
C. cars that release emissions D. emissions are released by cars

I do know that we omit “A & C” because after when we should use:

When + Subject + Verb, i.e. When he was a child, he used to cry a lot.
When + Noun (reduced form, i.e. When a child, he used to swim in this river.)
When + V –ing (“when” here is used as a preposition)
When + past participle (reduced form): i.e. When used wisely= when it is used), money can do a lot of things.

The answer is B. released from car emissions. i.e.:
Carbon dioxide may be absorbed by trees or water bodies, or it may stay in the atmosphere when it (= Carbon dioxide) is released from car emissions, while it is only in the atmosphere that chlorofluorocarbons find their home.
My question is that “why not “D. emissions are released by cars.” What’s wrong with it?
I’ll greatly appreciate your detailed answer.
Thanks in advance.
 
  • Glenfarclas

    Senior Member
    English (American)
    The problem with D is that it makes "when" sounds like it is simply describing two unrelated things as happening at the same time: "Carbon dioxide may stay in the atmosphere at the same time that emissions are released by cars." With B, there is the understood pronoun "it" which relates the phrase back to the subject of the sentence and makes it natural to understand the "when" as connecting a precondition and result: "Carbon dioxide, when it is released by cars, may stay in the atmosphere."

    Does that make sense?
     

    Parla

    Member Emeritus
    English - US
    Glen's technically right, but I think the difference is very subtle and it's a poor question to throw at an English learner; most reasonably well educated native speakers would find B and D equally acceptable.
     

    Lyndon

    Banned
    N/A
    Glen's technically right, but I think the difference is very subtle and it's a poor question to throw at an English learner; most reasonably well educated native speakers would find B and D equally acceptable.

    I agree with you, and I think it's a poor question for another reason, too. It suggests that "Carbon dioxide ... may stay in the atmosphere when ... released from car emissions" while the fact is that it can stay in the atmosphere when released from ANY source.
     
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