单项选择题
A．the agriculture revolution B．the invention of labor-saving machinery C．the development of scientific agriculture D．the farming machinery in America
A．The need for labor helped the invention of machinery in America B．The farmer rejected Charles New bolt's plow for fear of ruin of their fields C．Both Europe and America had great need for farmer machinery D．It was in Indiana that the first chilled-steel plow was produced
A．Europe B．America C．New Jersey D．Indiana