Answer: So, nitrogen is often the limiting factor for growth and biomass production in all environments where there is suitable climate and availability of water to support life.
Explanation:
You start by answering the first question on the list which tells you what to do from there. Pretend your animal was a bird, you might have a key like this:
Does this animal have wings?
yes go to question 2
no go to question 6
Then you just continue until the list doesn't say to do anything else. i.e.:
Is the animal feathered?
yes bird
no go to question 4
1. Nucleus 2. The smallest living thing are single cells and are the functional units of multicellular organisms. The answer could also be B