Because you cannot test it or observe it.
A scientific theory are created through the scientific method, in the scientific method you need to be able observe it and experiment.
Explanation:
The purines are ribose and deoxyribose
B is the answer for your question
The answer is A because cells take in glucose and oxygen and make energy, with the waste products of CO2 and H2O
i believe a stem cell is the only type of cell that can grow into any cell. meaning it can go anywhere it may be needed.