The correct answer is: Rat liver extract contains enzymes that may be necessary for certain mutagens to work.
Therefore, in the Ames test for mutagenicity, it is included in the control sample to ensure that any observed mutations are caused by the mutagen being tested and not by the absence of necessary enzymes. The mutagenicity of rat liver extract alone is tested by the control, while the mutagenicity of a chemical mutagen alone is tested in the second sample.
Rat liver extract contains enzymes that may be necessary for certain mutagens to work. Since it must be added to the experimental sample, it is also added to the control sample.
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