The Roomba 570 Is a Robotic Vacuum Cleaner Made and Sold

The Roomba 570 is a robotic vacuum cleaner made and sold by iRobot. It can vacuum 4 rooms on a single battery charge and it costs $10 worth of electricity to charge the Roomba. Lynn is a house cleaner and can vacuum 4 rooms in 1/2 hour with a traditional vacuum. Suppose you want the 4 rooms in your house vacuumed once a week for one year. Ignoring the cost of any electricity Lynn uses, which method is economically efficient if Lynn charges $20 per hour? $30 per hour?

A) Lynn if she charges $20 or $30 per hour
B) Lynn if she charges $20 per hour and Roomba if Lynn charges $30 per hour
C) Roomba if Lynn charges $20 per hour and Lynn if she charges $30 per hour
D) Roomba if Lynn charges $20 or $30 per hour