As many people hit middle age,they often start to notice that their memory and mental clarity are not what they used to be.We suddenly can’t remember___1___we put the keys just a moment ago,or an old acquaintance’s name,or the name of an old band we used to love.As the brain___2___,we refer to these occurrences as“senior moments.”___3___seemingly innocent,this loss of mental focus can potentially have a（n）___4___impact on our professional,social,and personal___5___.Neuroscientists,experts who study the nervous system,are increasingly showing that there’s actually a lot that can be done.It___6___out that the brain needs exercise in much the same way our muscles do,and the right mental___7___can significantly improve our basic cognitive___8___.Thinking is essentially a___9___of making connections in the brain.To a certain extent,our ability to___10___in making the connections that drive intelligence is inherited.___11___,because these connections are made through effort and practice,scientists believe that intelligence can expand and fluctuate___12___mental effort.Now,a new Web-based company has taken it a step___13___and developed the first“brain training program”designed to actually help people improve and regain their mental___14___.The Web-based program___15___you to systematically improve your memory and attention skills.The program keeps___16___of your progress and provides detailed feedback___17___your performance and improvement.Most importantly,it___18___modifies and enhances the games you play to___19___on the strengths you are developing—much like a（n）___20___exercise routine requires you to increase resistance and vary your muscle use.（6）选？
上下文语义衔接+固定结构辨析第二段第一句提到神经科学家们越来越显示人们可以做很多事。第二句提到“_____我们的大脑像肌肉一样需要锻炼……”。由此可推断第二句是第一句的详细阐释说明，因此，第二句是神经科学家们研究得出的结论。此外，根据动词搭配、用法可知，“It turns out that+句子”是常见的固定结构，表示“结果是，原来是，证明是”，详细阐述研究的具体结果。其他的选项都可以和out搭配，但是选项B和D的主语多为人而不是物。如果选项C放到空格处，则It的指代不明。因此只有A是正确答案，it作形式主语，真正的主语是后面that引导的从句。
