A worldwide ban on the production of certain ozone-destroying chemicals would provide only an illusion of protection. Quantities of such chemicals, already produced, exist as coolants in millions of refrigerators. When they reach the ozone layer in the atmosphere, their action cannot be halted. So there is no way to prevent these chemicals from damaging the ozone layer further. Which of the following, if true, most seriously weakens the argument above? ['The coolants in refrigerators can be fully recovered at the end of the useful life of the refrigerators and reused.', 'In modern societies, refrigeration of food is necessary to prevent unhealthy and potentially life-threatening conditions.', 'Even if people should give up the use of refrigeration, the coolants already in existing refrigerators are a threat to atmospheric ozone.', 'It is impossible to measure with accuracy the quantity of ozone-destroying chemicals that exist as coolants in refrigerators.']