In some parts of the United States, fanning is easy. But fanning has always been difficult in the northeastern corner of the country, which is called New England.
New England has many trees and thin, rocky soil. Anyone who has wanted to start a new farm there has had to work very hard. The first job has been cutting down trees. The next job has been digging stumps (树桩) of the trees out of the soil. Then the farmer has had the difficult job of removing stones from his land.
The work of removing stones never really ends, because every winter more stones appear. They come up through the thin soil from the rocks below. Farmers have to keep removing stones from the fields. Even today, farms which have been worked on for 200 years keep producing more stones.
That is why stone walls are used instead of fences around New England fields. The stone walls are not high; a man can easily climb over them. But they keep the farmer’s cows from joining his neigh bour’s COWS.In New England, stone walls are used().

A．to provide comfortable living conditions for the farmer’s family
B．to keep erous beasts out
C．to protect the farmer’s fields from thieves
D．to keep the farmer’s cows from wandering away