Which of the following, if true, most seriously weakens the argument above? A. Most people who suffer from osteoporosis are not aware that administration of fluoride can increase bone mass. B. Fluoride is added to drinking water in many locations in order to strengthen the teeth of people who drink the water. C. The risk of contracting osteoporosis and other degenerative bone diseases is lessened by exercise and an adequate intake of calcium. D. Unlike administration of fluoride, administration of estrogen or calcitonin is known to cause undesirable side effects for many people. E. The new bone mass that is added by the administration of fluoride is more brittle and less elastic than normal bone tissue.