【答案】A
【解析】第一段开头即点明了文章主旨：“the once，undisputed U．S．scientific lead…is diminishing．”。其他三段从三个方面对此作了阐述：“scientific training relatively unattractive to U．S．students”，“during the next few decades，this path likely will revert toward its more traditional length and reestablish， in a healthy way，the more traditional relationship…”，“another new force also greatly facilitated globalization”。由此可知，本文着重讨论的是科学全球化造成美国科技领先地位的下降。主旨归纳题，应明确文章的中心思想。