An electric power distributor charges residential customers $0.15 per kilowatt-hour (kWh). The company advertises that "green power" is available in 150 kWh blocks for an additional $5 per month. (Green power is generated from solar, wind power, and methane sources.)

a. If a certain customer uses an average of 480 kWh per month and commits to one monthly 150 kWh block of green power, what is her annual power bill?

b. What is the average cost per kWh with green power during the year?

c. Why does green power cost more than conventional power?