In the ames test, the appearance of his+ revertants in the presence of a non-mutagenic control compound indicates that the growth medium contains factors that are mildly mutagenic there is some low-level contamination in most experiments liver extract increases the potency of some mutagens some of the reversion mutations are not caused by the mutagen being tested

Option (4).

Explanation:

Ames test is used to determine the mutagenicity of any substance. The growth of bacteria in a particular chemicals determines the confirm mutagenecity of the chemical.

The his+ reverants are observed in mutagenic control, this may occur due to the reason that some reversion mutation is not caused by which the mutagen has been tested. This reversion mutation may have occur due to some other factors other than the mutagen.

Thus, the correct answer is option (4).

a- all cells come from pre-existing cells

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