Carbon and nitrogen are two substances that have parts of their cycles in which the substance must be "fixed". Thus, the correct option for this question is D.
A cycle may be defined as the series of steps that regularly run in a repeated manner to deliver the needs of organisms. Examples include the carbon cycle, the water cycle, etc.
Carbon is found as a solid in all of their isotopes but nitrogen is typically a gas that is present in 78% of the earth's atmosphere.
Option D is missing in the question which is carbon and nitrogen.
Therefore, the correct option for this question is D.
To learn more about the Carbon cycle, refer to the link:
https://brainly.com/question/2272536
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