In order to properly test what goes into a plant it is important to use a closed system
open system
natural environment

Explanation: Natural Environment i think this because it would be better for a plant to be in it's natural environment to actually be able to test it so it being in their natural environment would most likely be useful since you would be testing them in a state in which not only would be easier to test but also be the most useful to gather information from. I there stance my answer to be Natural Environment.

you can have just about every answer you can think of

the answer is c a tornado