Theories are used for: A. Testing hypotheses B. Investigating phenomenon C. Validating existing knowledge D. All of the above

Answer: D. All of the above

Explanation:

A scientific theory includes the explanation for the different aspects of cause of natural processes or events. These explanations are repeated tested and verified by utilizing the scientific methodologies. The scientific methodologies includes the making an observation, constructing a scientific question, drawing hypothesis, conducting experimentation, analyzing the data or results, and concluding the results.

A theory can be used for testing the hypothesis, investigating the cause of the natural phenomena, and give evidences for validating the knowledge of existing theories.