在有丝分裂的一个细胞周期中，最可能发生在同一时期的是（ ）

下图是“1955—2000年日本GDP增长率变化表”。下列关于图中圈出时段日本经济发展的描述正确的是(  )

设函数f（x）=ax²+bx+c且f（1）=﹣$\frac{a}{2}$，3a＞2c＞2b．

（1）试用反证法证明：a＞0

（2）证明：﹣3＜$\frac{b}{a}<-\frac{3}{4}$．

   Some people don’t like doing sports because they think it is hard work and will make____ (他们) feel tired . That’s wrong! Sports help us to____ (保持)healthy , happy and live longer .If you____ (从不) do sports , you will get unhealthy easily . Sports are____ (也)a good way to make friends .People from different____ (地方) may not know each other , but after a game , they often become good friends .And sports in winter and ____ (夏天)are not the same.____ __(swim) is fun in hot weather , while skiing is great____ cold days . Many people____ (享受) watching others play games .They often get very____ (兴奋) when “their” players or teams win .