The correct answer is letter A. The disk soaked in distilled water is the controlled variable because it is the only differential factor from all of the other variables which are all disinfectants. Controlled variables are elements that are not changed throughout the experiment. When something is unchanged it is better for scientists to analyze the relationships of other variables better.
Organism a and b is the to closest to be the same
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The answer is B, fossil fules
A.) An allele is a version of a gene that can be expressed as a phenotype.
