Today,only 4 million people live and work on farms in the United States,but they produce more than enough food for 200 million Americans.They are able to do this because modern agriculture depends more on up-to-date machinery and scientific methods than it does on old-fashioned knowledge and human labor.

These days,one man or woman spending an afternoon in an air-conditioned,stereo-equipped tractor can do the work that years ago would have required hundreds of hours of back-breaking drudgery.

As a result of the modernization of agriculture,the farm of today looks a lot like a factory.When the farmer's hens lay their eggs,conveyor belts carry the eggs off to be sorted and packed.If the farmer keeps pigs,those pigs will not be wallowing in the mud,but lying around in sanitary pens.And the farmer is likely to be found studying a computer print-out of long-range weather conditions.Why isn’t the farmer milking cows in the barn?The answer is easy:A machine is doing it.

This reliance on modern technology has its problems for farmers.The tractors that plow their fields,the combines that reap and thresh the grain,and the silos that store the grain for winter feeding are all very expensive.For example,a single tractor can cost as much as$50,000.Without this equipment,farmers could not run their farms profitably,but,to pay for it,farmers must take out large bank loans.A single bad season could put them deeply in debt.And,when the machinery wears out or becomes obsolete,the farmers must replace it with newer,more expensive models.Still,most farmers are willing to put up with these difficulties because their machines have freed them from the heavy labor and hard life that farmers had to endure in the past.

In the United States there are fewer farmers,because(　　).