What is an example of a closed system?
a.) an estuary where fresh and ocean water mix
b.) a living single-celled organism such as a phytoplankton
c.) a river that flows into the ocean
d.) a net constructed along a section of the river to separate the foreign fish species from the rest of the river

Correct B). A living single-celled organism such as a phytoplankton

A closed system is known to be the physical system that does not allow exchange of any matter or energy in and out of the system. It is an isolated system in which there is no interaction with its external environment.

A single cell organism act like a closed system as there is only one cell then there is no any exchange of matter or energy between them as in multi-cellular organism.

Hence, the correct answer would be option B.

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explanation:

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.