Suppose that average annual income (in dollars) for the years 1990 through 1999 is given by the linear function: $I(x)=1,054 x+23,286,$ 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 1990 s, average annual income increased by $\$ 1,054$
d. Average annual income rose to a level of $\$ 23,286$ by the end of 1999.