根据以下资料，回答下列各题。 As many people hit middle age, they often start to notice that

theirmemory and mental clarity are not what they used to be. We suddenly can’tremember ___1___ we put the keys just a moment ago, or an old acquaintance’sname, or the name of an old band we used to love. As the brain ___2___, werefer to these occurrences as “senior moments.” ___3___ seemingly innocent,this loss of mental focus can potentially have a (n) ___4___ impact on ourprofessional, social, and personal ___5___. Neuroscientists,experts who study the nervous system, are increasingly showing that there’sactually a lot that can be done. It ___6___ out that the brain needs exercisein much the same way our muscles do, and the right mental ___7___ cansignificantly 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___ mentaleffort. Now, a newWeb-based company has taken it a step ___13___ and developed the first “braintraining program” designed to actually help people improve and regain theirmental ___14___. The Web-basedprogram ___15___ you to systematically improve your memory and attentionskills. The program keeps ___16___ of your progress and provides detailedfeedback ___17___ your performance and improvement. Most importantly, it___18___modifies and enhances the games you play to ___19___ on the strengthsyou are developing—much like a(n) ___20___exercise routine requires you toincrease resistance and vary your muscle use. 请在第__1__处填上正确答案。

A.where

B.when

C.that

D.why