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Which sentence is correct? When do we use "what" and when do we use "which"?

-What number do they live at? 145a or 145b.
-Which number do they live at? 145a or 145b.
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It should be "which," I believe. Webster defines "which" in this way:

1 : being what one or ones out of a group -- used as an interrogative

It defines "what" like this:

1 a (1) -- used as an interrogative expressing inquiry about the identity, nature, or value of an object or matter (2) -- often used to ask for repetition of an utterance or part of an utterance not properly heard or understood b (1) archaic : WHO 1 -- used as an interrogative expressing inquiry about the identity of a person (2) -- used as an interrogative expressing inquiry about the character, nature, occupation, position, or role of a person

(Of course, I have omitted the irrelevant parts of the definitions, as I'm sure you have realised.)
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'Which' is used when we have to choose something among a small number of things.

(Pointing at the cars in front of your house)
Which car is yours? There is a small number of cars to choose from.

What type of car do you have?
I don't specify any number and there are many types of cars available.

Which number do they live at?
We use 'which' because there are only two options to choose from.
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Comments  
lol. You are great, your posts are fun. It just takes me ages to understand them.
Aww, thanks! In the future, though, I'll try to be a little less obtuse. How might I make myself better understood?
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By using feweR---->>>>---<<<<<<<---------------<<<<<<<<<<>>>>>>>>>>>-------.
 Woodward's reply was promoted to an answer.
Sorry about that. I copied and pasted from M-W's online site, and I didn't bother to get rid of all the extra stuff from the definitions. My apologies!
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