Final answer:
The problem with the sentence is the unclear antecedent for 'they'. It could refer to either the 'library' or 'the floor-to-ceiling bookshelves', causing confusion. A good solution would be using 'It' to directly reference the library.
Step-by-step explanation:
The issue with this sentence lies in the unclear antecedent of the pronoun 'they'. It's not quite clear what 'they' refers to in the sentence 'They would be the best room in the house!'. While the reader might deduce that 'they' refers to 'the floor-to-ceiling bookshelves', it could also be mistakenly assumed that 'they' refers to the 'library'. Therefore, to improve clarity, the second sentence could be rephrased as 'It would be the best room in the house!', where 'It' clearly refers to the library.
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