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At is your preposition, she is a pronoun, walked is your verb, and crowd is a noun.
The preposition in the sentence about the concert hall is B. At.
What is a preposition?
A preposition simply means a word that can be used before a noun or pronoun to show directly, time, place, etc.
Prepositions act to connect the people, objects, time, and locations of a sentence. They are usually short words and are normally placed directly in front of nouns.
Examples of prepositions include on, at, to, of, etc. They're also used to introduce an object.
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