The pet industry is big business in the United States. The amount spent yearly on pets continues to increase.U.S. Expenditure on Pets

Year	Expenditures (billion dollars)
1994	17
1996	21
1998	23
2001	28.5
2002	29.5
2003	32.4
2004	34.4
2005	36.3
2006	38.5
2007	41.2
2008	43.4

(a) Based on a scatter plot of the data in the table, why is a linear model appropriate? The scatter plot suggests a steady, ---Select--- quadratic increase linear decrease quadratic decrease linear increase .

(b) Align the data so that 1994 is year 0. Write the function for the linear model that gives the amount spent each year on American pets, where x is the year since 1994, with data from

0 x 14.

(Round all values to three decimal places.)

S(x) =

(c) Use the model to estimate the amount that will be spent on pets in 2014. (Round your answer to three decimal places.)

S(20) = billion dollars

What assumption(s) must be made for this to be considered a valid estimate?