The Ames Test Is Used to Measure the Mutagenicity of Molecules

The Ames test is used to measure the mutagenicity of molecules by introducing a potential mutagen to a culture of bacteria that cannot synthesize histidine. Which of the following explains the results if the bacteria are able to grow in a medium that lacks histidine?

A) all proteins expressed by the bacteria have mutated to not require histidine
B) defective histidine synthesis genes are mutated to forms that allow for histidine synthesis
C) the mutagen causes the bacteria to use some other molecule in place of histidine
D) the bacteria can convert the potential mutagen into histidine
E) none of the above