TOEFL Listening: ETS-TOEFL听力机经 - E9R75BMKO719V53NP$

What is the professor's attitude about the use of magnetic fields for steering nanovehicles? A. It has the advantage of allowing nanovehicles to steer themselves independently. B. It is an effective approach but other approaches are needed as well. C. It is a prone steering method but decreases engine efficiency. D. It has been the most difficult challenge so far in the development of nanovehicles.