A closed system is a system that makes no physical or chemical exchanges with its environment. Of these systems provided as choices (tree, human, clock, car), only a clock does not make exchanges with the environment, except for energy.
Therefore, of that list, only a clock would be a closed system.
Hope that helped! =)
Heart Blood vessels. (Arteries, Capillaries, and Veins)
Plants are producers and make their own food through a process of photosynthesis.
Organisms are classified into domains according to shared characteristics.
