TOEFL Listening: ETS-TOEFL听力机经 - N4NYWI1JB908OPT2P$

Why does the nanoengine that the professor discusses not need to carry fuel? A. The machine is pushed forward by air or water pressure. B. The machine is powered externally by rotating magnets. C. The carbon wheels on the machine rotate freely on their own. D. The liquid in which the machine is immersed is fuel supply.