TOEFL Listening: ETS-TOEFL听力机经 - 3W45PD35J5BMC9V1P$

Why does the professor mention honey? A. To help illustrate the difficulty of moving nano-vehicles through water or air. B. To explain why fluids like water have greater density than air. C. To indicate a surprisingly useful source of energy for nano-vehicles. D. To make a comparison between insect movement and the movement of none-vehicles.