A study found that adults who drink coffee regularly have a 2.5 times higher risk of heart disease than adults who drink less coffee. It can be concluded that a substance in coffee can cause heart disease in people. Which of the following most weakens the above conclusion? ['Coffee contains fly that improves heart vitality.', 'Drinking coffee during meals can lower blood lipids.', 'People at high risk of heart disease are more likely to fall in love with coffee.', 'Coffee lovers are mostly cheerful and like sports.']