We aren't interested in whether or not rats can get Alzheimer’s disease. This answer choice doesn't strengthen the link between Creutzfeldt-Jakob disease in rats and Alzheimer's in humans.

The way Creutzfeldt-Jakob disease affects the rats doesn't matter. We're interested in the virus that the disease came from.

This answer choice actually weakens the argument, because it rules out a possibility heavily implied by the stimulus: that the virus that causes Creutzfeldt-Jakob disease in rats is the same as the virus that causes Alzheimer's in humans. If C is true, then the virus that causes Creutzfeldt-Jakob disease in rats can't cause Alzheimer's in humans, because it has no effect on them.

If Creutzfeldt-Jakob disease and Alzheimer's disease are two manifestations of the same disease, and we know for sure that Creutzfeldt-Jakob disease is caused by a virus, then it makes sense that Alzheimer's is also caused by a virus. This strengthens the argument.

We're interested in drawing a conclusion about Alzheimer's disease, which we know affects humans. Knowing that blood from uninfected rats doesn't affect the test rats doesn't strengthen the link to our conclusion, which is based on human blood being injected into rats.