Rhizobium bacteria living in the roots of bean plants or other legumes produce fixed nitrogen, which is one of the essential plant nutrients and which for nonlegume crops, such as wheat, normally must be supplied by applications of nitrogen-based fertilizer. So if biotechnology succeeds in producing wheat strains whose roots will play host to Rhizobium bacteria, the need for artificial fertilizers will be reduced. The argument above makes which one of the following assumptions? ['Fixed nitrogen is currently the only soil nutrient that must be supplied by artificial fertilizer for growing wheat crops.', 'Rhizobium bacteria living in the roots of wheat would produce fixed nitrogen.', 'There are no naturally occurring strains of wheat or other grasses that have Rhizobium bacteria living in their roots.', 'Biotechnology should be directed toward producing plants that do not require artificial fertilizer.']