The average annual salary of the employees of a company in the year 2005 was ninety thousand dollars. It increased by the same factor each year and in 2006, the average annual salary was $91,200. Let f(x) represent the average annual salary, in thousand dollars, after x years since 2005. Which of the following best represents the relationship between x and f(x)?
f(x) = 91.2(1.013)x
f(x) = 90(1.013)x
f(x) = 91.2(2.2)x
f(x) = 90(2.2)x