今年5月18日．英美科学家公布了人类第一号染色体的基因测序图，这个染色体是人类“生命之书”中最长也是最后被破解的一章．据报道，第一号染色体中共有2.23亿个碱基对，2.23亿这个数用科学记数法可表示为（ ）
A．2.23×105
B．2.23×106
C．2.23×107
D．2.23×108
2.23亿=223 000 000=2.23×108．故选D．
完形填空(15分) Doctors tell us that holidays are important．We should have a __36__ from work for a week or two weeks every year．If it is possible，they say, we should __37__ our homes and go to other _38__ of the country．We should go away for __39__．Then，after the holidays，we’ll be _40__ and get ready for __41__ again． But some people do not want to leave their homes to __42__ in other places．They do not like a sudden change in their lives．They think their _43__ are best of all．And some people can have a rest in __44__ way．They can go to visit their friends．They can have __45_holidays together．
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