问答题 　　As people age, the brain changes in both good ways and bad.　　If you are over 20, your cognitive performance is probably alreadyon the wane. The speed over which people can process information【S1】____________ declines at a steady rate from as early as their 20s.　　A common test of processing speed is the "digit symbol substitution test", in that a range of symbols are paired with a set of numbers in a【S2】____________code. Participants are shown the code, given a row of symbols, then【S3】____________asked to write down the corresponding number in the box below within a set period. There is nothing cognitively challenging about the task; levels of education do no difference to performance. But age does.【S4】____________Speed consistently declines as people get older.　　Fortunately, there is some good news to go with the bad. Psychologists distinguish "fluid intelligence", which is the ability【S5】____________ to solve new problems, and "crystallized intelligence", which roughly equates to an individual’s stock of accumulating knowledge. 【S6】____________　　These reserves of knowledge continue to increase with age: people’s performance on vocabulary and general-knowledge tests keeps improving into their 70s. Yet experience can often 【S7】____________compensate for cognitive decline. In an old but instructive study of typists ranging in the age from 19 to 72, older workers typed【S8】____________just as fast as young ones, even though their tapping speed was【S9】____________slower. They achieved this by looking further ahead in the text, which allowed them to keep going smoothly.【S10】____________《问题》：【S4】