TOEFL Listening: ETS-TOEFL听力机经 - A6X6V11O9X_FK33KP$

According to the professor, what were two of the requirements for building an effective aqueduct? A. The water source could not be underground. B. Changes in water level had to be gradual. C. The city being served by the aqueduct had to be lower than the water source. D. The water source needed to be a maximum of 50 kilometers from the city.