Answer:
C. Interactions between a group of things
Explanation:
A system is made up of many parts. These parts combines together and gives the system its overall functionality.
The interaction between the different parts to attain a common goal or purpose is the most distinguishing feature of a system. In the human body, the circulatory system moves blood round the body in order to move food from one place to another and to provide energy. Some parts of the circulatory system functions to take the blood into the areas where they can undergo gaseous exchange, some goes to the kidney where wastes are removed e.t.c these interacting parts of the circulatory system gives its function.
