The scientist's hypothesis is not about finding aluminium in the brain, it was about the causal relationship between excess aluminium and elevated amyloid protein that results in Alzheimer. Option A suggests "some" healthy people has "unusually low" concentration of aluminium. "Unusually low" is the problem word here. If all the healthy patients had low aluminium that the argument would have been more sound. Moreover, this option does not target the main point of argument.Option E indicates the relationship between aluminium and Alzheimer like symptoms that corroborates authors arguments.