Why D is the correct answer?

Hi, we are trying to weaken the idea that since the change is tiny, genetic modification is harmless. D tells us that even a tiny change can have disastrous effects, directly undermining the argument. C only tells us that the location of certain genes are unknown, we have no idea if these genes whose locations are unknown are the same genes that are being modified

but D says it affects animals, and the stimulus says affects humans. also the change in toxic levels due to alteration, it does not say if the change becomes more toxic or less. it doesn't establish a starting point for the toxic level. C directly refutes the premise by saying the characteristic (the part thats genetically modified) does not exist basically to scientist? im confused how d is any better than c.

Humans are animals. Even ignoring that, the fact that one gene alone can alter a plants toxicity in some context clearly indicates that even minor alterations can have massive effects. The authors argument is premised on the idea that a slight change cannot have a big enough effect to worry about. This means we don't actually care whether the impact is positive or negative, the mere fact that it has a large effect directly refutes the last sentence. C doesn't refute anything. It says that scientists have not yet cataloged the location of every gene for every type of food. This is nothing like saying the genes do not exist.