TOEFL Listening: ETS-TOEFL听力机经 - I4MY31RVY536CK1WP$

What does the professor imply about the practice of assigning a monetary value to trees? A. It has motivated residents in some cities to plant a large number of trees. B. It has not yet been shown to have a positive economic impact on cities. C. It is motivated by the need to make decisions about spending money on planting or maintaining trees. D. It is a practice that involves too many factors for the result to be meaningful.