Which of the following is an example of an ecosystem? A. an oyster
B. a school of fish
C. fish and sharks
D. the open ocean

D. the open ocean

Explanation:

The reason why I cannot be any of the other answers are because, A is just a single organism which would in fact be incorrect. A school of fish would be incorrect because, it only includes a bunch of the same fish. The Ecosystem (As it says in the capture below) is a bunch of different organisms that work together like a shark might eat the fish but, that is about it. The entire ocean is a great example of a ecosystem because, there are all kinds of things that create the ecosystem. Therefore, your answer could only be D. the open ocean.

Information in the picture below:

cancer   is   essentially   uncontrolled   mitosis   of specialized cells

the supermassive black hole is in the middle of the galaxy.