Answers and Explanations
1 . D Postindustrial economies, in which agriculture, mining, and industry are highly productive,
feature relatively large numbers of employment opportunities in service industries.
2 . C Radar was first used in World War II. The other answer choices were initially used in World
War I.
3 . A The Helsinki Accords called for communication between Communist and non-Communist
nations, making (C) an incorrect response. They did not address nuclear disarmament (B) or
space flights (D).
4 . D Railroads were a nineteenth-century invention, whereas the remaining response choices were
developed in the twentieth century.
5 . D Twentieth-century developments included nuclear weaponry. They also included household
appliances (A). At the end of the twentieth century, the United States and the Soviet Union
cooperated in space dockings and an international space station (C). Radio was more popular in
the United States than in Europe (B).