The monthly water bill is a function of the number of gallons used. The cost is ​$9.10 for 10,000 gal or less. Over 10,000 ​gal, the cost is ​$9.10 plus​ $1 for each 1000 gal over 10,000 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?