The greatest contribution to civilization in the century may well be the air-conditioning- and American leads just as amazing is the speed with which this situation came to be. Air-conditioning began to spread in industries as a production aid during World War II. Today most Americans need to take air-conditioning for granted to homes, offices, factories, theatres, shops, studios, schools, hotels, and restaurants, and we can see it almost everywhere in today's America. But not everybody is aware that high cost and easy comfort are merely two of the effects of the vast cooling of American. In fact, air conditioning has substantially altered the country's character and customs. Many of the byproducts are so conspicuous  that they are scarcely noticed. To begin with, air-conditioning transformed the face of America by making possible those glassy, boxy, sealed-in skyscrapers. It has been indispensable, no less, to the functioning of sensitive advanced computers, whose high operating temperatures require that they be constantly cooled. . . It has, at will, forced families into going back into families with closed doors and shut windows to improve the effects of the air-conditioning, reducing the interactions of neighborhood life. I think there is no need explaining the seriousness of this consequence . It is really surprising that the public's often noted withdrawal into self-pursuit and privacy has kept in pace with the historic spread of air-conditioning. Though science has little studied how habitual air-conditioning affects mind and body, some medical experts suggest that, like other technical avoidance of natural changes in climate, air-conditioning may damage the human capacity to adapt to stress. If so, air-conditioning is only like many other greatly useful technical developments that liberate  man from nature by increasing his productivity and power in some way - while indirectly weakening him in others. According to this passage, which of the following played a role in the forming of the unique character of U.S.?
Its advanced computerized civilization. Its excessive use of air-conditioning. Its public's retreating into self-pursuit. Its greatest contribution to human civilization.