单选题
In 1970, the United States was still by far the most productive of countries in the world, with a gross national product of $ 1, 014, 750 million which amounted to $ 4,950 per person.
【正确答案】
D
【答案解析】worked up to be：逐渐达到……；worked in being：参加……； worked on being：继续工作……；worked out to be：算出，总共达……。题中 amount to意为“总计……”，可知D选项最符合题意要求。