下图是某同学在观察洋葱根尖细胞有丝分裂时拍摄的显微照片，下列叙述正确的是

在图中，直线与圆弧分别表示两质点、从同一地点出发，沿同一方向作直线运动的图。已知的初速度，它的速率由变为所花时间为。

（1）试求在任意时刻的加速度；
（2）设在停止时，恰好追上，求的加速度；
（3）在什么时候，、的速度相同？

丁二酮（）可用作糖果增香剂．请计算：
（1）丁二酮是由_________元素组成的．
（2）碳元素和氢元素的质量比为______．
（3）丁二酮的相对分子质量为，则的数值为____．
（4）的丁二酮含碳元素的质量为____ ．

    A century ago, the immigrants from across the Atlantic included settlers and sojourners. Along with the many folks looking to make a permanent home in the United States came those who had no intention to stay, and who would make some money and then go home. Between 1908 and 1915, about 7 million people arrived while about 2 million departed. About a quarter of all Italian immigrants, for example, eventually returned to Italy for good. They even had an affectionate nickname, “uccelli di passaggio,” birds of passage.
    Today, we are much more rigid about immigrants. We divide newcomers into two categories: legal or illegal, good or bad. We hail them as Americans in the making, or brand them as aliens to be kicked out. That framework has contributed mightily to our broken immigration system and the long political paralysis over how to fix it. We don’t need more categories, but we need to change the way we think about categories. We need to look beyond strict definitions of legal and illegal. To start, we can recognize the new birds of passage, those living and thriving in the gray areas. We might then begin to solve our immigration challenges.
    Crop pickers, violinists, construction workers, entrepreneurs, engineers, home health-care aides and physicists are among today’s birds of passage. They are energetic participants in a global economy driven by the flow of work, money and ideas. They prefer to come and go as opportunity calls them. They can manage to have a job in one place and a family in another.
    With or without permission, they straddle laws, jurisdictions and identities with ease. We need them to imagine the United States as a place where they can be productive for a while without committing themselves to staying forever. We need them to feel that home can be both here and there and that they can belong to two nations honorably.
    Accommodating this new world of people in motion will require new attitudes on both sides of the immigration battle. Looking beyond the culture war logic of right or wrong means opening up the middle ground and understanding that managing immigration today requires multiple paths and multiple outcomes, including some that are not easy to accomplish legally in the existing system.