Which of the following, if true, most seriously weakens the author's argument? A. Plastic composite fillings are more expensive than silver fillings. B. Most patients are satisfied with the durability of their silver fillings. C. A patient's exposure to mercury from multiple silver fillings is at a harmless level. D. Most dental insurance plans do not consider the replacing of silver fillings to be necessary. E. Mercury is also present in certain seafoods, alcoholic beverages, and medications.