Generic medicines contain the same active ingredients, in the same quantity, as the original brand-name medicines for which they are designed to be substituted. Nonetheless, there are sometimes important differences in the effects produced in a patient by generic medicines as compared to the brand-name equivalents. Which of the following, if true, most helps to resolve the seeming discrepancy described above? ['Because producers of generic medicines are not involved in research and development their products can be sold at lower prices.', 'The inactive ingredients and fillers, which differ in different versions of a medicine, can affect the rate at which the medicine is absorbed and its concentration in the bloodstream.', 'When the patent on an original brand-name medicine expires, federal law permits the production of a generic version without further studies on the effectiveness of the active ingredients.', 'Older persons, who are more likely to require prescription medicines, are more sensitive than younger persons are to slight variations in dosage levels.']