Suppose that average annual income (in dollars) for the years 1990 through 1999 is given by the linear function: I(x) = 1,054x + 23,286, where x is the number of years after 1990. Which of the following interprets the slope in the context of the problem?

a. As of 1990, average annual income was $23,286. 

b. In the ten-year period from 1990–1999, average annual income increased by a total of $1,054.

c. Each year in the decade of the 1990s, average annual income increased by $1,054.

d. Average annual income rose to a level of $23,286 by the end of 1999.