One-year-olds ordinarily prefer the taste of sweet food to that of salty food. Yet if one feeds a one-year-old salty food rather than sweet food, then over a period of about a year he or she will develop a taste for the salty flavor and choose to eat salty food rather than sweet food. Thus, a young child' s taste preferences can be affected by the type of food he or she has been exposed to. Which one of the following is an assumption required by the argument? ["A child's taste preferences usually change between age one and age two.", 'Sweet food is better for infant development than is salty food.', 'Two-year-olds do not naturally prefer salty food to sweet food.', 'The salty food fed to infants in order to change their taste preferences must taste pleasant.']