21. When the supply of a given resource dwindles, alternative technologies allowing the use of different resources develop, and demand for the resource that was in short supply naturally declines. Then the existing supplies of that resource satisfy whatever demand remains. Among the once-dwindling resources that are now in more than adequate supply are flint for arrowheads, trees usable for schooner masts, and good mules. Because new technologies constantly replace old ones, we can never run out of important natural resources.
Which one of the following, if true, most seriously undermines the conclusion?
(A) The masts and hulls of some sailing ships built today are still made of wood.
(B) There are considerably fewer mules today than there were 100 years ago.
(C) The cost of some new technologies is often so high that the companies developing them might actually lose money at first.
(D) Dwindling supplies of a natural resource often result in that resource's costing more to use.
(E) The biological requirements for substances like clean air and clean water are unaffected by technological change.