What part of the oxygen cycle is missing from this diagram? A. Weathering causes the release of oxygen stored in sediment.
B. Land plants release oxygen as part of photosynthesis.
C. Organic materials such as seashells fall to the ocean bottom and their oxygen is incorporated into sediment.
D. Aquatic living organisms use oxygen dissolved in water for respiration.

Aquatic living organisms use oxygen dissolved in water for respiration.

Explanation:

In this diagram, we can see very well how the cycle of oxygen works and what is involved in it. One thing is missing from it though, that being the aquatic living organisms and their role in the cycle. We only have a circulation of oxygen between the ocean and the atmosphere presented, but no living organisms are depicted.

The aquatic living organisms are crucial for the oxygen cycle, as they actually have a much larger contribution than the terrestrial living organisms. The amount of oxygen that is found in the ocean is far greater than the one on land. It is the aquatic organisms that actually managed to make the biggest contribution to the changes in the atmosphere, which enabled life on land later.

81 is a composite number.