Answer: at, through.
Explanation: a preposition is a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause. In the given sentence, there are two prepositions: "at" which object is "the concert hall" and "through" (through can be a preposition, an adverb or an adjective, but when it is followed by a noun, it is a preposition) which object is "crowd."
