TOEFL Listening: ETS-TOEFL听力机经 - 43BLGJ5B151WM14LP$

According to the professor what purpose is served by the waxy layer of the Nepenthes pitcher? A. If attracts insects to enter the plant. B. It produces the plant's digestive enzymes. C. If causes insects to slide down info the fluid in the pitcher. D. It secretes a sticky substance that keeps trapped insects from escaping.