Why are stains such as methylene blue used when observing cells under the microscope?

What types of energy transfers and energy transportations are involved in the domino chain reaction?

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Which of the following statements is true about the relationship between genetic variation and natural selection? a. genetic variation must be present in a population before natural selection can act on it. b. genetic variation arises in a population as a result of natural selection. c. natural selection can act on a population whether there is genetic variation within the population or not. d. after natural selection acts on a population, the amount of genetic variation in the population always increases.