单选题 A great change in agriculture in the 19th century included two things: the invention of labor-saving machines and the development of scientific agriculture. Labor-saving machines naturally appeared first where more labor was needed. "In Europe," said Thomas Jefferson, "the object is to make good use of their land, labor being abundant(充裕的); here it is to make good use of our labor, land being abundant." It was in America, therefore, that the great advances in the 19th century agricultural machines 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 tools on their backs; by 1860, most of the machines in use today had been designed in an early form. One of the most important 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, would have none of it, declaring that the iron poisoned the soil and made the weeds grow, however, 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 Which of the following is the main topic of the passage?

单选题 The word "naturally" (Line 3, Para. 1) is closest in meaning to ______.

单选题 From what Thomas Jefferson said, it can be inferred that ______.

单选题 We can infer that the word "here" (Line 4, Para. 1) refers to ______.

单选题 ______, the first chilled-steel plow made their appearance.