Animal feed should not include genetically modified plants. A study found ... ...

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?


(A)  Potatoes are not normally a part of the diet of laboratory rats. 

(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in PT71 (Dec 2013): Logical Reasoning A