有如下四个命题： ①平面α和平面β垂直的充要条件是平面α内至少有一条直线与平面β
题型：填空题
问题：
有如下四个命题：
①平面α和平面β垂直的充要条件是平面α内至少有一条直线与平面β垂直；
②平面α和平面β平行的一个必要不充分条件是α内有无数条直线与平面β平行；
③直线a与平面α平行的一个充分不必要条件是平面α内有一条直线与直线a平行；
④两条直线平行是这两条直线在一个平面内的射影互相平行的既不充分也不必要条件．
其中正确的序号是______．
题型：填空题
阅读理解。
Plants are very important living things. Life could not go on if there were no plants. This is because
plants can make food from air, water and sunlight. Animals and men can't make rood from air, water
and sunlight. Animals get their food by eating plants and other animals. Therefore, animals and men need
plants in order to live. This is the reason why we find so many plants around us.
If you look carefully at the plants around you, you will find that there are many kinds of plants. Some
plants are large while,others are small. Most plants are green.
Some plants have many small leaves. Others have only a few. There are many kinds of leaves.
Leaves are of different shapes and colors. Leaves are also different in other ways. Some leaves have
long thin parts at the end, we call them leaf stalks, some have short leaf stalks, others even have no leaf
stalks. The leaf edges are also different. Some leaves have smooth (平滑) edges. Some have a wave-shaped
edges. Others have edges like the teeth of a saw (锯子).
1. Which of the following shows the right relation (关系) according to the first paragraph? [ ]
2. Plants are important because . [ ]
A. we need to eat plants every day
B. men can't live without plants
C. we can see so many plants around us
D. animals eat plants as their only food
3. We know from the passage that . [ ]
A. all leaves have leaf stalks
B. all leaves have smooth edges
C. plants are different in shapes but have the same color
D. leaves are different in shapes and colors
4. The underlined word "leaf stalks" in the last paragraph means . [ ]
A. 叶边
B. 叶纹
C. 叶柄
D. 叶尖
5. The main idea of the passage is . [ ]
A. how plants help men and animals
B. plants are important and they are quite different
C. plants are more important than air, water and sunlight
D. why we find so many plants around us