What is the professor's opinion of the potential of helium-3 as an energy source? A. Mining helium-3 on the Moon will eventually be easy. B. Continuing to extract helium-3 from the Moon is not worthwhile. C. Helium-3 will probably be a significant energy source within a few years. D. Using helium-3 on a large scale will not be practical until nuclear fusion reactors are built.