In recent years the climate has been generally cool in northern Asia. But during periods when the average daily temperature and humidity in northern Asia were slightly higher than their normal levels the yields of most crops grown there increased significantly. In the next century, the increased average daily temperature and humidity attained during those periods are expected to become the norm. Yet scientists predict that the yearly yields of most of the region' s crops will decrease during the next century. Which one of the following, if true, most helps to resolve the apparent paradox in the information above? ['Crop yields in southern Asia are expected to remain constant even after the average daily temperature and humidity there increase from recent levels.', 'The climate in northern Asia has generally been too cool and dry in recent years for populations of many crop insect pests to become established.', 'Any increases in temperature and humidity would be accompanied by higher levels of atmospheric carbon dioxide, which is vital to plant respiration.', 'In many parts of Asia, the increased annual precipitation that would result from warmer and wetter climates would cause most edible plant species to flourish.']