After 1950, in response to record growth in worldwide food demand, farmers worldwide sharply increased fertilizer use. As a result, the productivity of farmland more than doubled by 1985. Since 1985, farmers have sought to increase farmland productivity even further. Nevertheless, worldwide fertilizer use has declined by 6 percent between 1985 and the present. Which one of the following, if true, most helps to resolve the apparent discrepancy in the information above? ['After fertilizer has been added to soil for several years, adding fertilizer to the soil in subsequent years does not significantly improve crop production.', 'Several varieties of crop plants that have become popular recently, such as soybeans, are as responsive to fertilizer as are traditional grain crops.', 'Between 1950 and 1985 farmers were able to increase the yield of many varieties of crop plants.', 'Between 1975 and 1980 fertilizer prices temporarily increased because of labor disputes in several fertilizer-exporting nations, and these disputes disrupted worldwide fertilizer production.']