Which of the following, if true, most seriously weakens the argument above? A. Just one mining company controls all the ytterbium that is imported into the United States, where these medical tools are manufactured. B. The process of extracting pure ytterbium from its ores, euxenite and xenotime, is very expensive and involves strong acids. C. Ytterbium is also used as an additive, in small quantities, in the manufacture of stainless steel and solid-state lasers. D. Some common alloys of tin also increase their electrical resistance under mechanical loads. E. The largest source of ytterbium is in a relatively remote region of China.