A dog enthusiast took home two puppies from the dog shelter. He fed one of the puppies super-premium canned dog food, while he fed the other a generic brand from the grocery store. The dog fed on the generic brand gained weight twice as fast as the dog fed the super-premium brand. Because of the difference in these results, the dog enthusiast concluded that the generic brand actually provided nutrition superior to that of the super-premium brand.
Each of the following, if true, would weaken the evidence for the dog enthusiast’s conclusion except for which of the following ？（）
A. A dog will sometimes gain more weight when eating inferior-quality food because he must eat more of it to obtain the nutrients he needs.
B. The dogs were of mixed breeds and appeared to be descended from dogs of different sizes.
C. Both dogs ate all of the food given to them at each serving.
D. The dog enthusiast did not give the two dogs equal amounts of their respective foods.
E. The dog who received the super-premium dog food suffered from a digestive system disorder that hindered his growth.