Scientists, puzzled about the development of penicillin-resistant bacteria in patients who had not been taking penicillin, believe they have found an explanation. The relevant group of patients have dental fillings made of mercury-containing amalgam, and the bacteria the patients develop are immune to mercury poisoning. Scientists have concluded that the genes causing resistance to penicillin are closely bundled on the chromosomes of bacteria with the gene that produces immunity to mercury poisoning. Exposure to the mercury kills off bacteria that lack the relevant immunity gene, and leaves room for those that possess both the mercury-immunity gene and the penicillin-resistance gene to flourish. Which one of the following most accurately characterizes the role played in the passage by the unstated assumption that some patients who take penicillin develop bacteria with an immunity to penicillin? ["It is the tentative conclusion of previous research that appears to be falsified by the scientists' discovery of the mechanism by which bacteria become resistant to mercury poisoning.", 'It is a generalization that, if true, rules out the possibility that some people who do not take penicillin develop bacteria resistant to it.', 'It is a generalization assumed by the scientists to conclusively prove that the explanation of their problem case must involve reference to the genetic makeup of the penicillin-resistant bacteria.', 'It is a point that, in conjunction with the fact that some patients who do not take penicillin develop penicillin-resistant bacteria, generates the problem that prompted the research described in the passage.']