制备乙酸乙酯时，饱和碳酸钠的作用：____．

使用显微镜观察洋葱鳞茎表皮细胞，在光线较暗时，应该选用（  ）

    When you take a walk in any of the cities in the West, you can see a lot of people ____(walk) with dogs. It is still true that a dog is the most useful and faithful animal in the world, but the reasons why people keep a dog ____(change). In the old days people used to train dogs to protect themselves against attacks by other beasts. And ____(late) they came ____(realize) that a dog was not only useful for protection,____ willing to obey his master. For example, when people used dogs for hunting, the dogs would not eat what was caught without ____(permit). But now people in the city need not protect themselves against attacks of animals. Why do they keep dogs, then? Some people keep dogs to protect themselves from robbery. But the ____(important) reason is for companionship. For a child, ____ dog is his best friend when he has no friends to play with. For young couples, a dog is ____child when they have no children. For old couples, a dog is also their child when their real children have grown up. So the main reason why people keep dogs has changed from protection ____ friendship.

贵州FAST望远镜是目前世界第一大单口径射电望远镜，反射面总面积约250000m²，这个数据用科学记数法可表示为____．