I am pretty sure that the answer is B! I just took the Test and it makes sense because abiotic means not living feature and gravel isn't a living feature! Hope this helps!
<span>b. gravel on the bottom of the aquarium</span>
An ecosystem involves both the biological (plants, animals, human beings) and non-biological (land, water, soil, and atmosphere) community which interacts as a system. More importantly, the living things are very dependent on the abiotic community since it cannot survive by itself. Every animal, plant and human needs the primary physiological needs of water, food and shelter provided by the abiotic system. <span> </span> Biotic factors: Plants Microorganisms Macroorganisms
