A common genetic mutation that lowers levels of the enzyme cathepsin C severely reduces a person' s ability to ward off periodontitis, or gum disease. The enzyme triggers immunological reactions that destroy diseased cells and eliminate infections in the mouth. But researchers are developing ways to restore the enzyme to normal levels. Once that happens, we will be able to eliminate periodontitis. Which one of the following is an assumption on which the argument depends? ['A person whose cathepsin C level has been restored to normal will not suffer from periodontitis.', 'Genetic mutation is the only cause of lowered levels of cathepsin C.', 'Researchers will soon succeed in finding means of restoring cathepsin C to normal levels.', 'Persons who do not have the genetic mutation that lowers levels of cathepsin C do not get gum disease.']