Advertisement for a lactase supplement: Lactase, an enzyme produced by the body, aids in the digestion of lactose, a natural sugar found in dairy foods. Many subjects in an experiment who drank a liter of milk on an empty stomach showed signs of lactose intolerance -- difficulty in digesting dairy products because of insufficient lactase. Thus, extrapolating from the number of subjects adversely affected, at least 50 million people in North America alone should take lactase supplements. Which one of the following statements, if true, most seriously weakens the argument? ['Lactose intolerance can interfere with proper nutrition.', 'Some dairy foods, such as cheese, contain a form of lactose more difficult to digest than that found in milk.', 'Most people who consume dairy products consume less lactose at each meal than the amount found in a liter of milk.', 'Eating solid food when drinking milk can decrease the amount of lactase produced by the body.']