It is implied but not stated in the passage that__________．

逻辑推断题。从文章第二段的The most important of the early inven-tions was the iron plow可以看出B是明确表明了的；从这句Nevertheless，many people de-voted their attention to the plow来看，C也是原文给出了的；从这句Charles Newbolt of New Jersey had been working on the idea of a cast--iron plow and spent his entire fortune in intro-ducing his invention可以得知D也是原文明确表明的；从第一段here it is to make the most of our labor，land being abundant(充分利用劳动力，土地资源丰富)，可以推断出劳动力缺乏，所以A是文章暗含的内容。