It is one of nature's great ironies that the availability
of nitrogen in the soil frequently sets an upper limit on
plant growth even though the plants'leaves are bathed
in a sea of nitrogen gas. The leguminous plants—among
them crop plants such as soybeans, peas, alfalfa, and
clover—have solved the nitrogen supply problem by
entering into a symbiotic relationship with the bacterial
genus Rhizobium.
Which of the following situations is most closely analogous to the situation described by the author as one of nature's great ironies?
答案：C