Some doomsayers are warning that long-range warming or cooling trends in weather patterns will drastically reduce grain production. More optimistic reports, however, point out that, even if such drifts in average temperature do occur, we should expect little change in grain production because there is little evidence that changes in rainfall patterns will occur. Moreover, for most crops, climate-induced yield trends will be masked by both the year-to-year fluctuation of yields and by the enhancement of yields because of technological factors. Which of the following is an assumption on which the more optimistic reports mentioned in the passage are based

A. Long-range changes in weather patterns cannot be accurately predicted.
B. The growing of grain is so highly dependent on technological factors that improvements in yield are unlikely, regardless of climatic conditions.
C. Trends in rainfall patterns are more difficult to isolate than are trends in temperature.
D. Long-range warming or cooling trends are more damaging to grain production if they are accompanied by changes in rainfall patterns than if they are not.
E. Long-range cooling trends are potentially more destructive to grain production than are long-range warming trends.