Environmentalist: When bacteria degrade household cleaning products, vapors that are toxic to humans are produced. Unfortunately, household cleaning products are often found in landfills. Thus, the common practice of converting landfills into public parks is damaging human health. Which one of the following is an assumption the environmentalist's argument requires? ['Converting a landfill into a public park will cause no damage to human health unless toxic vapors are produced in that landfill and humans are exposed to them.', 'If vapors toxic to humans are produced by the degradation of household cleaning products by bacteria in any landfill, then the health of at least some humans will suffer.', 'In at least some landfills that have been converted into public parks there are bacteria that degrade household cleaning products.', 'If a practice involves the exposure of humans to vapors from household cleaning products, then it causes at least some damage to human health.']