What does the professor imply about using carbon nanotubes in the development of space elevators? A. Current technology is good enough to make space elevators even without nanotubes. B. We do not yet have the technology to bind nanotubes together in a ribbon. C. Nanotube cables would not be rigid enough to support an elevator car. D. Nanotubes are the kinds of materials that will be needed if space elevators are ever to be built.