Windmills are devices or machines which are specifically designed to use the wind force and convert it into power that humans could use in many ways. Windmills have been used for may different purposes for many centuries and countries around the world.
In the mid to late 19 centuries, most windmills in the Great Plains were used to pump water. B is the correct answer. This water was primarily for humans and animal consumption. In fact, most of them are still been used nowadays for the same purpose.
The types of windmills used in the Great Plains were mainly of the self governing type, which allowed the machine to regulate its own speed during high winds. This was and important feature at the time, because it kept the entire structure protected, from being destroyed by the generation of centrifugal forces during the operation in high wind conditions.