A biologist develops a simulation of the carbon cycle in a laboratory by setting up a closed ecosystem. The ecosystem consists of a glass container with an Earth-like atmosphere and a body of freshwater. In the water are a fish and an aquatic plant. The container is kept in well-lit and temperature controlled conditions. Which of the following best explains how the fish influences the carbon level in the atmosphere?

A. It increases the carbon dioxide levels through cellular respiration.
B. It decreases the carbon dioxide levels by consuming the plant.
C. It decreases the carbon dioxide levels when it decomposes.
D. It increases the carbon dioxide levels through photosynthesis.