Which of the following is an example of a closed system?
an estuary where fresh and ocean water mix
a chemical reaction within deep ocean organisms allowing them to glow
a living single-celled organism such as a phytoplankton
a river that flows into the ocean

The answer for the given question above would be the second option. The one that is an example of a closed system is a chemical reaction within deep ocean organisms allowing them to glow. By definition, a closed system is a system that doesn't exchange any matter with its surroundings. Hope this answers your question.

Because it's sugar that us the answer