The extremely high incidence of birth defects and cancer among children in Tellersville, Nevada, has long been suspected to be linked to the nuclear-weapons facility in which nearly three-quarters of the town's population works. It now appears, however, that the town's water supply, tainted by the discharges of a nearby plastics factory, is to blame for the town's health problems. Scientists have studied two groups of rodents. One group was exposed to the same level of radiation present at the nuclear-weapons facility but given pure water. The other was exposed to no radiation but given tainted water from Tellersville. The latter group showed an incidence of cancer and birth defects ten times higher than normal and six times higher than that of the pure water group.

Which one of the following, if true, most undermines the conclusion that Tellersville's water supply is responsible for the town's health problems?