Amanda exercised for 10 minutes every day in the first week, 20 minutes in the second week, 30 minutes in the third week, and 40 minutes in the fourth week.

Billy exercised for 10 minutes every day in the first week, 20 minutes in the second week, 40 minutes in the third week, and 80 minutes in the fourth week.

Which statement best describes the methods used by Amanda and Billy to increase the time they spent exercising?

Amanda's method is linear because the number of minutes increased by an equal number every week.
Billy's method is linear because the number of minutes increased by an equal factor every week.
Both Billy's and Amanda's methods are exponential because the number of minutes increased by an equal factor every week.
Both Billy's and Amanda's methods are exponential because the number of minutes increased by an equal number every week.