Members of the Amazonian Akabe people commonly take an early-morning drink of a tea made from the leaves of a forest plant. Although they greatly enjoy this drink, at dawn they drink it only in small amounts. Anthropologists hypothesize that since this tea is extraordinarily high in caffeine, the explanation for the Akabe' s not drinking more of it at dawn is that high caffeine intake would destroy the surefootedness that their daily tasks require. Which one of the following, if true, most seriously calls the anthropologists' explanation into question? ['The leaves used for the tea contain a soluble narcotic.', "The Akabe also drink the tea in the evening, after their day's work is done.", 'The drink is full of nutrients otherwise absent from the Akabe diet.', 'When celebrating, the Akabe drink the tea in large quantities.']