Which determiner, which or what, to use for some situations, like these:
What month is it? What school are you attending now?
or
I understand that which" expresses the idea that in the context of the sentence there is a list of possibilities to chose from.
"what" is more open-ended.
However, can there be a better distinction?
In interrogative usage: What Identifies and which Choses.
As you understand it, what is open-ended in the sense that a large number of possibilities can be the answer.
When the possibilities are a smaller number/quite known, consider using "which".
Generally, they are not interchangeable, in some instances you can replace "which" with "what". There needs to be a context of choice. For example:
Which/What flavor of ice cream would you like?
•Either is fine, but "which" is better.
The explanations that have been given are similar to those that you'll find in many style guides. However, many native speakers do not seem to be aware of what has been prescribed. You'll often see and hear 'what' when the rules demand 'which'.
I tell learners that it's not very important. If they choose the 'wrong' word, few people will notice.
From what I understand, "which" is supposed to be used to denote informative clauses that may or may not be necessary; at least, in a statement, not a question.
For instance:
I've got a hold of the fire extinguisher in order to control this fire, which has already become a raging inferno.
As far as questions go, "which" is normally used when there's a choice that needs to be made.
Which flavor of ice cream would you like?
"What" is normally used when the choices are infinite, or the speaker is trying to ask for some kind of information.
What's the fourth Wonder of the World?
Both of the sentences you mentioned in the original post should most likely use "which."
