What is a common criticism raised by Kurzweil's critics regarding nanobot advancement?

• A. Where will these nanobots get their energy?
• B. Why would they be as big as suggested?
• C. Why would they be able to think for themselves?
• D. How will they be manufactured?

Answer: (A)

Powering nanobots is the big hurdle critics focus on. Even at nanoscale, devices need a reliable energy source to operate, move, sense, and communicate. The problem is that energy density and safe delivery inside a living body (or in the outside environment) don’t currently scale well to billions of tiny machines. Batteries at that scale are impractical, and alternative power methods—like harvesting energy from the body or from external sources—face serious limits in duration, safety, and practicality. If you can’t provide a feasible and safe energy supply, all other capabilities—navigation, manipulation, autonomy—can’t function long enough to be useful. While concerns about manufacturing or potential intelligence of the systems are relevant, the energy question is the most fundamental and widely emphasized obstacle.