Answer: Possession of Valves
Explanation: Veins possess valves which ensures that blood flows in only one direction. Valves helps blood to flow back to the heart against gravitational force.
Arteries don't have valves because the pressure originating from the heart is capable of causing the blood to flow in one direction.
Only a fraction of the energy<span> available at one trophic </span>level<span>is </span>transferred to the next<span> trophic </span>level<span>; the fractions can vary between 1-15%, with an average value of 10%. Typically the numbers and biomass of organisms decreases as one ascends the </span>food chain<span>.</span>
