Answer: D. (-infinity, 2)
This is the interval [tex]-\infty < x < 2[/tex]
On this interval, the function curve is going downhill as you read it from left to right.
More technically, the function is decreasing on this interval because f(a) > f(b) where a < b. For example, f(0) > f(2) and note that 0 < 2.
