Which number is the main idea?
(1)Technology revolutionized agriculture as inventions dramatically increased productivity on the farms. (2)Eli Whitney’s invention of the cotton gin in 1793 permitted an individual to clean three hundred pounds of cotton in a single day—three hundred times more than could be cleaned by hand. (3)After the mechanization of wheat farming, the hours required to farm one acre dropped from sixty-one to three, and the per-acre cost of production fell from $3.65 to $0.66. (4)Machines entered every phase of agriculture—by 1890 some 900 companies were manufacturing such items as hay loaders, cord binders, seeders, rotary plows, mowers, and combines.