Ecologist: Smallpox, one of the worst diseases ever to afflict humans, has in some outbreaks killed as much as 50 percent of local populations. The last known surviving cultures of variola -- the smallpox virus -- are confined to two high-security laboratories. Some scientists are anxious to destroy the remaining variola cultures, to which humans are susceptible. Research on the cultures, however, may someday lead to important benefits for humans, and so the cultures should not be destroyed. Which one of the following, if true, would do most to strengthen the ecologist's argument? ['Smallpox has killed millions of humans over the centuries, and when it reaches the stage of an epidemic it is extremely difficult to eradicate.', 'It is becoming increasingly important to prevent any nation from acquiring the means to wage biological warfare.', 'It is more likely that the virus, if left available to researchers, will lead to an important medical breakthrough than that the virus will be accidentally released from the laboratory.', 'It is likely that the virus, if released, will develop a resistance to vaccines previously used to control it.']