Concept:Certain soil microorganisms help crops grow by improving nutrient availability.Explanation:Some bacteria are beneficial to crops.For example, Rhizobium bacteria live in the root nodules of legumes and fix atmospheric nitrogen for plant use.Decomposer bacteria also break down organic matter and release essential nutrients into the soil.Nematodes, viruses, and protozoa are generally harmful or not directly useful to crops.Answer:B. Bacteria