All of the rabbits, deer, and several bird species found within a field are considered to be part of a/an
a. biosphere.
b. population.
c. ecosystem.
d. community.

I agree that the answer is community

just FYI:

Community: deals with groups of organisms living or somehow interacting with one another in the same field/habitat.

Population: deals with same species of animals in a group in a place

Ecosystem: deals with all living organism and where they are at in an environment.

Biosphere: deals with the area in which there is life on this planet and integrates all living species and their relationship with one another and their interactions with other  parts of the planet such as the atmosphere/ecosystems.

Hopefully this helped and good luck!! If you need any clarifications just let me know.  

the answer to your question is:

c. complementary base-pairing

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