Which of the following best accounts for the difference in mutagenic activity between PAHs and PAHs plus liver preparation?

a. in the absence of liver preparation, the bacteria metabolized the polynuclear aromatic hydrocarbons into mutagenic agents
b. in the presence of the liver preparation, histidine could not be synthesized by the bacteria
c. the enzymes in the liver preparation resulted in the metabolic activation of the polynuclear aromatic hydrocarbons into a carcinogenic chemical
d. the exposure of the bacteria to human cellular components resulted in the back-mutation of the polynuclear aromatic hydrocarbons