Given a scientific passage and an answer, generate a question for the given answer.

Q: Passage: Acidity is measured on a pH scale. Rain that is 5.0 or less on that scale is considered acid rain. Answer: ph scale

A: Acidity is measured on what kind of scale?
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Q: Passage: A population is a group of individuals of the same species that live in the same area. The population is the unit of natural selection, adaptation, and microevolution. In ecology, how large a population is and how quickly it is growing are often used as measures of a species' health. Answer: population

A: What term refers to a group of individuals of the same species that live in the same area and is the unit of natural selection, adaptation, and microevolution?
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Q: Passage: Gaseous nitrogen is converted into forms that can be used by plants during the process of nitrogen fixation. Answer: nitrogen fixation

A:
Gaseous nitrogen is converted into forms that can be used by plants during a process called what?
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