In some parts of the United States, farming is easy. But farming 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 the stumps of the trees out of the soil. Then the farmer has had the difficult job of removing stones from his land.     This 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 neighbor's cows. This selection says that the work of removing stones   _  .
must be done again and again is usually done during the winter was more difficult before machines were used is easy after machines were used
Question: In some parts of the United States, farming is easy. But farming 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 the stumps of the trees out of the soil. Then the farmer has had the difficult job of removing stones from his land.     This 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 neighbor's cows. This selection says that the work of removing stones   _  .
A. must be done again and again
B. is usually done during the winter
C. was more difficult before machines were used
D. is easy after machines were used
Answer: