Which statement describes the rate of change of the following function?
f(x) = 9x - 4
a.
the function has a constant rate of change, decreasing for all x at a rate of 4.
b.
the function has a constant rate of change, increasing for all x at a rate of 9.
c.
the function has a varying rate of change when x > 9.
d.
the function has a varying rate of change when x < 4.