A doctor investigated whether an insufficient amount of dopamine in the brain causes Parkinson' s disease. The doctor injected dopamine into the bloodstreams of patients with Parkinson' s disease. The progress of the disease was not halted, nor did any of the patients improve. The doctor concluded that Parkinson' s disease must be caused by something other than a lack of dopamine in the brain. Which of the following, if true, casts the most doubt on the doctor's conclusion? ['Recent research has shown that dopamine, when injected into the bloods, cannot enter the brain.', "Research has shown that dopamine often causes diseases other than Parkinson's disease.", "Parkinson's disease causes the brain to produce less dopamine than it normally would.", 'Research has shown that many different chemicals interact to influence the workings of the brain.']