Given f of x is equal to the quantity x plus 6 end quantity divided by the quantity x squared minus 9x plus 18 end quantity, which of the following is true?
A- f(x) is decreasing for all x < 6
B- f(x) is increasing for all x > 6
C- f(x) is decreasing for all x < 3
D- f(x) is increasing for all x < 3
