TOEFL Listening: TPO-TOEFL听力TPO - K6W1E88NZX1CT99ER$

What is the main purpose of the talk? A. To explain the mechanical functioning of barrages. B. To discuss some possible ecological effects of building barrages. C. To discuss the effects of ocean tides on coastal ecosystems. D. To describe ways to increase biological productivity of estuaries.