Jim, Tom and Tim are in the park. It's a small park, but it's nice. They can see some animals there. There are some birds in the tree. They are yellow. They can fly high. Quack… Quack… What do you hear? Oh，Some ducks. They can swim in the water. There are seven white ducks and nine black ducks. What's that? It's a cat. It's under the tree. It's white. It can jump very fast. The children like the small animals very much.

下图为用普通光学显微镜观察某植物根尖细胞有丝分裂过程中的两个视野，其中细胞甲为主要观察对象，若要使视野A变为视野B，下列有关的操作项目一般不需要的是（　　）

据资料统计：在明后期至清前期200余年间，世界白银产量的一半流入中国，拥有一流城市、最为密集和完善市场网络的中国，成为当时世界经济和贸易的中心区域。然而当时它却没有形成强大的扫荡旧经济基础的革命性变化。其内在的原因在于

一个物体由静止开始做匀加速直线运动，加速度大小为a₁，经 $\frac{t}{2}$ 时间后，物体的加速度大小变为a₂，又经过t时间恰好回到出发位置，则a₁与a₂的大小之比为（   ）