Before the mid－nineteenth century，people in the United States ate most foods only in season.Drying，smoking，and salting could preserve meat for a short time，but the availability of fresh meat，like that of fresh milk，was very limited;there was no way to prevent spoilage.But in 1810a French inventor named Nicolas Appert developed the cooking－and－sealing process of canning.And in the 1850s an American named Gail Borden developed a means of condensing and preserving milk.Canned goods and condensed milk became more common during the 1860s，but supplies remained low because cans had to be made by hand.By 1880，however，inventors had fashioned stamping and soldering machines that mass－produced cans from tinplate.Suddenly all kinds of food could be preserved and bought at all times of the year.
Other trends and inventions had also helped make it possible for Americans to vary their daily diets.Growing urban populations created demand that encouraged fruit and vegetable farmers to raise more produce.Railroad refrigerator cars enabled growers and meat packers to ship perishables great distances and to preserve them for longer periods.Thus，by the 1890s，northern city dwellers could enjoy southern and western strawberries，grapes，and tomatoes，previously available for a month at most，for up to six months of the year.In addition，increased use of iceboxes enabled families to store perishables.An easy means of producing ice commercially had been invented I the 1870s，and by 1900the nation had more than two thousand commercial ice plants，most of which made home deliveries.The icebox became a fixture in most homes and remained so until the mechanized refrigerator replaced it in the 1920s and 1930s.
Almost everyone now had a more diversified diet.Some people continued to eat mainly foods that were heavy in starchesorcarbohydrates，and not everyone could afford meat.Nevertheless，many families could take advantage of previously unavailable fruits，vegetables，and dairy products to achieve more varied fare.
20.What does the passage mainly discuss？
Causes of food spoilage.
Commercial production of ice
Inventions that led to changes in the American diet.
Population movements in the nineteenth century.
21.The phrase in seasonin line 2refers to
a kind of weather
a particular time of year
an official schedule
a method of flavoring food.
22.The word preventin line 4is closest in meaning to
estimate
avoid
correct
confine
23.During the 1860s，canned food products were
unavailable in rural areas
shipped in refrigerator cars
available in limited quantities.
A staple part of the American diet.