In the oxygen–carbon dioxide cycle, a. animals produce oxygen used by plants for photosynthesis.
b. animals produce oxygen used by plants for respiration.
c. animals produce carbon dioxide used by plants for photosynthesis.
d. animals produce carbon dioxide used by plants for respiration.

C

Explanation:

Animals produce carbon dioxide as a byproduct of cellular respiration. Plants use the carbon dioxide through photosynthesis to produce oxygen.

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.