The Ames Test Is Used to Test the Mutagenicity of a Compound

The Ames test is used to test the mutagenicity of a compound suspected to be a carcinogen. In a simple form of the test, the carcinogen is first mixed with a rat liver extract. A disc of filter paper is soaked with this mixture and placed on a culture of a strain of Salmonella typhimurium that is defective in a gene involved in the synthesis of histidine, an amino acid that is essential for cell growth and proliferation. The strain is thus normally unable to grow into visible colonies when the histidine in the culture medium is depleted. In the presence of a mutagen, however, mutations (often "reverse mutations" in the same gene) can enable the bacteria to produce histidine on their own, and therefore grow into colonies. The results of the Ames test for three compounds A, B, and C-each used at the same concentration-are shown in the schematic diagram below. Colonies are indicated with black dots, and the disc is indicated with a white circle at the center of each plate. Which compound (A to C) appears to be a stronger mutagen in this assay? Write down A, B, or C as your answer.

A)
B)
C)