TOEFL Listening: ETS-TOEFL听力机经 - 1GHW1J9E9Y33HV9QP$

What does the professor say about the social benefits of trees? A. They are less important than other benefits. B. They can be difficult to quantify. C. They have only recently been acknowledged by city planners. D. They are more obvious to most people than are other benefits of trees.