c) the Articles of Confederation
The Articles of Confederation became the first "law of the land" for the United States in 1781, 4 years after it was created. However, it was not very effective. In 1787 plans were made to revise it, but politicians decided to do away with the Articles, creating the US Constitution, instead.
The unit is called Ecosystem.
Ecological units comprise concepts such as population, community and ecosystem.
<em>An ecosystem</em> is a community of plans and animals interacting with each other in an particular area. They also interact with their non-living environments (water, earth, sun, soil, climate and atmosphere).
The robins are in relationship with the earthworms because the robins eat them in order to survive and as a part of the food chain. The food chain is a linear sequence of organism through which nutrients and energy pass as one organism eats another. Ecosystems consist of creatures that mutually benefit from each other.
The answer to your question is A
