Why is the term linchpin appropriate in describing the termites

The term linchpin is appropriate in describing the termites as they are ecologically important for ecosystem. They play different roles.Termites being decomposers digest the cellulose present in the decaying plant matter, they convert cellulose into reusable form for plants. Termites fix nitrogen by digesting the nutrients in the soil and separates the nitrogen from the attached atoms which is easily taken up by the plants. Termites creates shelter for various insects colonies. They burrow deep inside the soil, creates tunnels suitable for insects colonies living.    

i believe it would cause an increase of percipitation, though i could be wrong. so i would say b is the answer.

he forelimbs below belong to four species that descended in a direct line from the ancestral mammal shown above. of the four descendants shown, which species most likely appeared earliest in evolutionary history?