What or which

[Sajumon]

What part of a country are you from?
Instead of 'what' do we need to use 'which'?
If both are correct , what is the difference in the meaning of the questions?

[Mister Micawber]

Which part of the country are you from?-- 'Which' is used when the speaker is considering a very limited set of 'parts'
What part of the country are you from?--'What' is used when the speaker is thinking that there are many parts to the country.
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Native English teacher at Mister Micawber's