In a study, shoppers who shopped in a grocery store without a shopping list and bought only items that were on sale for half price or less spent far more money on a comparable number of items than did shoppers in the same store who used a list and bought no sale items. Which one of the following, if true, most helps to explain the apparent paradox in the study's results? ['The grocery store in the study carries many expensive items that few other grocery stores carry.', 'The shoppers who did not use lists bought many unnecessary items.', 'Usually, only the most expensive items go on sale in grocery stores.', 'Only the shoppers who used a list used a shopping cart.']