TOEFL Listening: ETS-TOEFL听力机经 - U3JB1EEF3511L4XJP$

Why does the professor mention insulin? A. To give an example of a protein that is used as medicine B. To point out the most common protein produced from plant seeds C. To give an example of a medication that can be stored for a long period D. To explain why it is difficult to extract proteins from plants