この at はどういう使われ方をされていますか。
Native speaker’s answer
Rebecca
Yes, it is necessary to put "at" before "which" here. The preposition "at" is used in this sentence because it forms a prepositional phrase with the object of the preposition "which". "Which" introduces a relative clause, and the prepositional phrase "at which" combines two sentences into a single sentence. Ex: I went to a store at which sold winter clothes. Ex: This is the hospital at which I work.