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It looks like you have a passage with some questions related to it. I can help you with that. Let's start by answering the questions. 26. The correct answer is A. "a" because "most serious problems" does not specify a particular problem, so we use the indefinite article "a." 27. The correct answer is B. "however" because it shows contrast between the increasing number of factories and the disadvantages of living in a big city. 28. The correct answer is C. "polluted" because it describes the air, which has been affected by smoke from factories. 29. The correct answer is A. "that" because it introduces a defining relative clause explaining the factors causing air pollution. 30. The correct answer is B. "diseases" because the sentence talks about serious health issues caused by air pollution. Now, let's move on to the second part of your request: To complete each sentence, I need to see the sentences and words in capital letters that need to be put into the correct form to fit into each gap. Feel free to provide me with more information so I can assist you further!