Which statement concerning an ecosystem is correct?

Considering the following statements;
A. It can exist without a constant source of energy input
B. It must contain consumers but can exist without producers.
C. It involves interactions between biotic and abiotic factors.
D. It can exist on land, but it cannot exist in lakes, rivers, or oceans.
The correct statement is C. that an ecosystem involves interactions between biotic and abiotic factors. 
Biotic factors are the living things in an ecosytem, they include plants, animals, bacteria, fungi and others. Abiotic factors on the other hand, are the non-living factors in an ecosystem, they include water and air, sunlight, the amount precipitation in an ecosystem is also an example of abiotic factor. The two types of factors are crucial for an ecosystem to exist and thrive. 

Very hard to understand