The monthly water bill is a function of the number of gallons used. The cost is $19.60 for 20,000 gal or less. Over 20,000 gal, the cost is $19.60 plus $1 for each 1000 gal over 20,000 with any fraction of 1000 gal charged at a fraction of $1. On what interval is the cost constant? On what interval is the cost increasing? On what interval is the cost constant? Select the correct choice below and, if necessary, fill in the answer box to complete your choice.
A. The cost is constant on the interval(s)
(Type your answer in interval notation. Use a comma to separate answers as needed.)
B. The cost is never constant.