阅读理解。

    One day a tiger catches(抓住)a fox(狐狸)in the forest. Before the tiger could eat him up,the fox cries out, “You can't eat me. Don't you know that I'm king(王)of the forest. If you eat me up, all the other animals in the forest will be angry with you. ”The tiger doesn't believe him. “How can such a small animal be king of the forest?" He asks himself. “If you don't believe(相信)me, ”the fox says, “then take a walk with me in the forest. You can see for yourself whether(是否)they are afraid of me. ”The tiger thinks it is a good idea, so the fox walks in front and the tiger goes behind. When the other animals see the tiger coming, they run away as fast as they could.  

   “See for yourself, Mr. Tiger,” he calls out quickly. “All the animals are afraid of me. ”

   “Yes, yes,”the tiger says. “You are quite right." He then lets the fox go.

农村常常建横截面积为半圆形的全封闭塑料薄膜蔬菜大棚，如图所示，如图不考虑塑料薄膜接头重合及埋在土里的部分，那么搭建一个这样的蔬菜大棚需要用塑料薄膜的面积是（ ）

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如图表示人体干细胞发生的变化过程．下列有关此图的叙述，正确的是（　　）

我们把力的大小、方向和____ 叫做力的三要素．