Animals with a certain behavioral disorder have unusually high levels of aluminum in their brain tissue. Since a silicon-based compound binds to aluminum and prevents it from affecting the brain tissue, animals can be cured of the disorder by being treated with the compound. The argument is based on which one of the following assumptions? ['Aluminum is the cause of the disorder rather than merely an effect of it.', 'Aluminum is never present in normal brain tissue.', 'Introducing the compound into the brain tissue has no side effects.', 'Animals with the disorder have unusually high but invariable levels of aluminum in their brain tissue.']