Canned goods and condensed milk became more common during the 1860's, 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.
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