单选题 Passage 1: The agricultural revolution in the nineteenth 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 abundant; here it is to make the most of our labor, land being abundant". 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 a crude plow, 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 early 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 and spent his entire fortune in introducing his invention. The farmers, however, were not interested in it, claiming that the iron poisoned the soil and made the weeds grow. Nevertheless, many people devoted their attention to the plow, until in 1869 James Oliver of South Bend, Indiana, turned out the first chilled-steel plow.

单选题 What is the main topic of the passage?

单选题 The word "naturally" in line 3 is closest in meaning to ________.

单选题 Which of the following can be inferred from what Thomas Jefferson said?

单选题 What point is the author making by stating that farmers could carry nearly all their tools on their backs?

单选题 When was the iron plow invented?

单选题 The word "it" in line 12 refers to ______.