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 1810 a French inventor named Nicolas Appert developed the cooking-and-sealing process of canning. And in the 1850's an American named Gail Borden developed a means of condensing and preserving milk.
20. What does the passage mainly discuss?
(A) Causes of food spoilage
(B) Commercial production of ice
(C) Inventions that led to changes in the American diet
(D) Population movements in the nineteenth century