Actually, today is your lucky day! I found an examples for 室内 functioning as a prepositional phrase "in a/the room". 入住室內時可選擇有機果汁 (A seasonal Fruit basket upon arrival in the room) & 例如，室內只有一個手提滅火器。(There was, for example, only one hand fire extinguisher in the room.) [context.reverso.net]
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student456
Okay, thank you, I understand and accept the explanation.