During a recent period of time compact discs sales have been declining. The sales (billions of dollars) can be modeled by the function (f)t= -9/7t+17.9, where T is the number of years since 2005. Suppose sales decreased at twice the rate. How can you transform F to model the sales? a. vertically shrink by a factor of 1/2

b. vertically stretch by a factor of 2

c. horizontally shrink by a factor of 1/2

d. horizontally stretch by a factor of 2