Answer:
The answer is letter A.
Explanation:
The amount that each company charges per hour on the pool is known, however the amount of hours they will be working is not, and is needed to distinguish which company is cheaper. Therefore, the number of hours is substitued by x and it is right next to the amount they charge for each hour. To find what each company charges, you must sum up what they charge and what they charge per hour, times the hours they will have to work on it( which is unknown and by x).