The author discusses naturally occurring Pseudomonas fluorescens bacteria in the first paragraph primarily in order to do which one of the following? A. prove that increases in the level of such bacteria in the soil are the sole cause of soil suppressivity B. explain why yields increased after wheat fields were sprayed with altered Pseudomonas fluorescens bacteria C. detail the chemical processes that such bacteria use to suppress organisms parasitic to crop plants, such as wheat, sugar beets, and potatoes D. provide background information to support the argument that research into the agricultural use of genetically altered bacteria would be fruitful E. argue that crop rotation is unnecessary, since diseases brought on by phytopathogens diminish in severity and eventually disappear on their own