describe the critical processes by which nitrogen is cycled through the biotic and abiotic components of earth’s ecosystems and identify the primary storage sink for nitrogen.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.

it's c (engineer's do their work with mechanical stuff not on any living being)