听力原文：The agricultural revolution in the 19th century involved two things: the inventio

听力原文： The agricultural revolution in the 19th century involved two things: the invention of labor-saving machinery and the development of scientific agriculture. Labor-saving machinery naturally appeared first where labor was scarce. "In Europe." said Thomas Jefferson, "the object is to make the most of their land, labor being rich enough; here it is to make the most of our labor, land being rich enough." It was in the United States, therefore, that the great advances in nineteenth-century agricultural machinery first came.

At the opening of the century, with the exception of crude equipment, farmers could have carried practically all of the existing agricultural implements on their backs; by 1860, most of the machinery in use today had been designed in an early form. The most important of the inventions was the iron plow. As early as 1790 Charles Newbold of New Jersey had been working on the idea of a cast-iron plow andspent his entire fortune in introducing his invention. The farmers, however, were not interested in it, believing that the iron poisoned the soil and made the weeds row. Nevertheless, many people devoted their attention to the plow, until in 1869 James Oliver of South Bend, Indiana turned out the first steel plow.

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A．The invention of machine that can save labor and the progress of scientific agriculture.

B．The invention of crude equipments and agricultural implements.

C．The advance of agriculture machinery and the iron plow.

D．The idea of a cast-iron plow and steel plow.