Soybeans Will Have Several Hundred Rhizobium Nodules

Soybeans will have several hundred Rhizobium nodules.A study in Minnesota found that soybeans grown without ammonium sulfate fertilizer produced 52.4 bushels per acre,while adding 75 lbs of ammonium sulfate per acre gave a yield of 54.2 bushels per acre.What is the best explanation for this observation?

A) Rhizobium produces PO₄ and NH₄, so you don't need any fertilizer
B) Rhizobium produces NH₄, so you don't need N containing fertilizer
C) Rhizobium produces PO₄, so you don't need P containing fertilizer
D) Rhizobium cannot produce NH₄, so you need to add N containing fertilizer
E) Rhizobium cannot produce PO₄, so you need to add P containing fertilizer