The correct answer is: A. More than 7.
The alkaline regression stage is the final stage of sludge digestion. During this stage, the pH of the sludge increases as the organic matter is broken down. The pH of the sludge can reach values of 8 or 9. This is because the breakdown of organic matter produces ammonia, which is a base.
The other options are incorrect because they do not reflect the pH of the sludge during the alkaline regression stage. Option B is incorrect because the pH of the sludge is more than 7, not less than 7. Option C is incorrect because the pH of the sludge is more than 6, not less than 6. Option D is incorrect because the pH of the sludge is more than 7, not less than 7.
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