Which of the following, if true, best explains how improvements in pesticide use have been accompanied by greater losses to certain pests? A. Some dangerous but relatively ineffective pesticides common 40 years ago are no longer in widespread use. B. As pesticides have become increasingly pest-specific, controlling certain pests with pesticides has turned out to cost more in many cases than the value of crop losses caused by those pests. C. Because today's pesticides typically have more specific application conditions than did pesticides in use 40 years ago, today's farmers observe their fields more closely than did farmers 40 years ago. D. Certain pest-control methods that some farmers use today do not involve the use of chemical pesticides but are just as effective in eliminating insect pests as those that do. E. Forty years ago, much less was known about the effects of pesticides on humans and other mammalian species than is now known.