如图，在边长为2的正方形内部，以各边为直径画四个半圆，则图中阴影部分的面积是（　　）

阅读下面短文，在空白处填入一个适当的词，或填入括号中所给单词的正确形式。

    In recent years,sandstorms ____(occur) in China frequently,especially in northwestern areas, ____severe influence has caused wide concern in China.

There're several reasons for this.First of all,people are cutting down trees without planting more, ____(make) some land become deserts.Another reason is ____it rains less due to the environment pollution.

    To ____(efficient) prevent sandstorms,first,we should plant more trees and grass which can keep the soil ____ being washed away.Second,more green fences should ____(build) along the desert, ____it helps to control the process of the desertification.

    Let's take action to do what we can ____(prevent) sandstorms from today.Soon the golden days we have been looking forward to ____(come).

“红灯停、绿灯行、黄灯亮了等一等”是基本的交通规则．当绿灯亮起时，我们迈动双脚快速穿过马路，此反射活动的类型及参与的神经中枢分别是（ ）