Animal feed should not include genetically modified plants. A study found that laboratory rats fed genetically modified potatoes for 30 days tended to develop intestinal deformities and a weakened immune system, whereas rats fed a normal diet of foods that were not genetically modified did not develop these problems. Which one of the following, if true, most weakens the argument? ['The researchers conducting the study were unable to explain how the genetic modifications of the potatoes would have caused the intestinal deformities or a weakened immune system in the rats.', 'Potatoes are not normally a part of the diet of laboratory rats.', 'The rats tended to eat more of the genetically modified potatoes at the beginning of the 30 days than they did toward the end of the 30 days.', 'Intestinal deformities at birth are not uncommon among rats bred in laboratory conditions.']