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Pneumonia is an infection of the lungs ______ the lungs fill with fluid, leading to difficulty in breathing.

Should the blank be completed with "which" or "in which"? And can you explain why?

Note: This is a question from a test. I think the answer should be "which" but answer key says "in which" and I want to learn why.

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    Can you explain where this sentence came from? I'm wondering why you couldn't say "an infection which fills the lungs with fluid" instead.
    – J.R.
    Nov 22 '15 at 21:57
  • This is a question from a test. I think the answer should be "which" but answer key says "in which" and I want to learn why.
    – Pufu Kuff
    Nov 22 '15 at 22:14
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    That is critical information you should include in your question. Here's a page that explains why, and here's another one that might help, too. I'll go ahead and edit this question, but next time you'll know to include it. Welcome to ELL!
    – J.R.
    Nov 22 '15 at 22:19
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    "in which" is correct. See ell.stackexchange.com/questions/14591/…
    – MaxW
    Nov 22 '15 at 23:44
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    Seems awkward to me, given "the lungs fill with fluid in an infection of the lungs" doesn't seem like the right preposition. I would lean towards "with" instead of "in".
    – BobRodes
    Nov 23 '15 at 1:35
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Given a choice between "which" and "in which", "in which" is less wrong. 

Better options exist.  I would consider options such as "whereby", "due to which", "during which" or even "from which" as more reasonable than "in which". 
 

However, the point of the test question is that "which" by itself does not work. 
 

On its own, "which" is a pronoun.  Specifically, it's a relative pronoun, so it must refer to something earlier in the sentence.  In theory, it could refer to either "lungs" or "infection".  In this case, it refers to "infection", because "lungs" appears again in the subordinate clause.  We can't make much sense of "lungs filling lungs". 

The pronoun has to play a role in its clause.  It could be a subject, an object, or a complement.  The clause has the verb "fill", so we should check those roles in relationship to that verb. 

The verb "fill" doesn't license complements, so we can't even examine that option.  It's simply not available. 

If "which" were a subject, it would be immediately followed by the verb.  The clause in question would look like "which fills the lungs with fluids".  The blank is in the wrong place for this, so "which" can't be a subject. 

If it were the verb's object, it could precede the subject and the blank is in the right place: "which the lungs fill with fluid."  However, this would mean the lungs fill the infection with fluid.  Although that's grammatically fine, it simply doesn't make sense. 

An infection isn't the kind of thing that can be filled with fluid.  Lungs are tangible, concrete, and able to act as containers.  An infection is intangible, abstract, and unable to hold anything. 
 

All that's left is for "which" to be the object of something else.  In the phrase "in which", the "which" is the object of a preposition. 

In this test question, the verb "fill" doesn't offer "which" any sensible role to play.  The preposition "in" does offer a role, and the prepositional phrase "in which" can play the role of a modifier of the verb "fill". 

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