Why is alternating current more effective at long–distance travel than direct current? a. the power lines are made out of conductors. b. transformers increase or decrease voltage as needed. c. the ac generator is more efficient. d. ac has lower voltage than dc.

The use of alternating current is cheaper and more effective than direct current. This is due to the fact that long-distance electricity transport is regulated with the use of electrical transformers, which allows a wide range of voltages.

In this sense, an electrical transformer can increase or decrease the voltage in an electric circuit while maintaining power.

Therefore the correct option is B.