Final answer:
The given function, which is a horizontal line over the domain 0 ≤ x ≤ 20, does not decrease on any interval within this domain as the slope is zero everywhere. Thus, there are no intervals on which this function is decreasing.
Step-by-step explanation:
The student has asked to determine the interval(s) on which a given function is decreasing. The provided intervals are (−∞,1)∪(0,∞), but these intervals are not immediately helpful, given the description of the function, which is a horizontal line over the domain 0 ≤ x ≤ 20.
A function represented by a horizontal line does not increase or decrease; its rate of change is zero. This means that the derivative of the function, which measures the slope at any point along the function, is equal to zero everywhere on its domain. Since the topic seems to include intervals that are larger than the domain of the function (stated as 0 ≤ x ≤ 20), there may be a misunderstanding or a typo in the question.
To precisely answer the question: a horizontal line function does not decrease or increase on any interval within its domain; it remains constant. Hence, within the given domain of 0 ≤ x ≤ 20, the function f(x) is neither increasing nor decreasing.