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What or which : which one should be used?

  • What/which medicine is good for boosting immunity of children?

  • What/which is the longest river in the world?

I think both are correct. But I am not sure.

2 Answers 2

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Which is used when there is some defined set of possible answers.

What is used when there is not such a set.

Where the set is very large, either may be acceptable. Both of your examples are very large sets, unless the context restricts that set in some way.

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  • When the set is undefined can ”which” safely replace “what”? Commented Mar 2, 2019 at 14:57
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    If there's a set that someone could sit down and make, even if it would take a very long time, you can use which. So all the medicines in the world is such a set, all the stars in the sky are such a set, and so on. If you have to infer what the set it completely, it's not. So, "what would you like for dinner", if there's no options implied by context (or even if there is), but "which would you like for dinner" requires context to indicate what the set of options is. "Which meal would you like for dinner" works semantically even without that context, but is not idiomatic without it.
    – SamBC
    Commented Mar 2, 2019 at 14:59
  • if the context has been introduced and we know that there is a strictly defined set of possible options, can we use "what"? (even if we know that "which" would be more natural) Commented Mar 2, 2019 at 15:28
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    You can use what, but in that context it makes it seem more informal.
    – SamBC
    Commented Mar 2, 2019 at 16:09
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To answer your question I would say:

Which medicine is good for boosting immunity of children?

Which is the longest river in the world?

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