A survey was recently conducted among ferry passengers on the North Sea. Among the results was this: more of those who had taken anti-seasickness medication before their trip reported symptoms of seasickness than those who had not taken such medication. It is clear, then, that despite claims by drug companies that clinical tests show the contrary, people would be better off not taking anti-seasickness medications. Which one of the following, if true, would most weaken the conclusion above? ['Given rough enough weather, most ferry passengers will have some symptoms of seasickness.', 'The seasickness symptoms of the people who took anti-seasickness medication would have been more severe had they not taken the medication.', "The clinical tests reported by the drug companies were conducted by the drug companies' staffs.", 'People who do not take anti-seasickness medication are just as likely to respond to a survey on seasickness as people who do.']