Answer:
A They harnessed the power of water to operate.
B They required little labor to run.
Explanation:
The Gristmills is a system whereby the local farmers brought their grains such as corn, millet etc to a locally setup mill in-order to turn it into floor. This floor is then used to make bread thereby serving their families as a staple food.
In return the operator of the local mill get what is called a millers toll which more like his wage for the efforts expended in producing the corn. This method is popular in most of the Muslim countries in the 8th century. And, also, it tends to produce enough floor for the making of bread which serves the whole populace.
