Bacteria that benefit human beings when they are present in the body are called commensals. The bacterium Helicobacter pylori plays a primary role in the development of stomach ulcers. But since stomach ulcers occur in less than 10 percent of those harboring H. pylori, and since it allegedly strengthens immune response, many scientists now consider it a commensal. But this is surely misguided. Only about 10 percent of the people who harbor Mycobacter tuberculosis -- a bacterium that can cause tuberculosis -- get sick from it, yet no one would call M. tuberculosis a commensal. Which one of the following, if true, most seriously undermines the argument's reasoning? ['There are more people who harbor M. tuberculosis than people who harbor H. pylori.', 'People who harbor M. tuberculosis derive no benefit from its presence.', 'Stomach ulcers caused by H. pylori and tuberculosis can both be effectively treated with antibiotics.', 'Cases of tuberculosis usually last longer than ulcers caused by H. pylori.']