设计凸轮机构，当凸轮角速度、从动件运动规律已知时，则( )
A．基圆半径越大，压力角就越大
B．基圆半径越小，压力角就越大
C．滚子半径越小，压力角就越小
D．滚子半径越大，压力角就越小
参考答案：B
解析： 本题主要考查的知识点为凸轮机构的压力角与基圆半径的关系。
[要点透析] 设计凸轮时，基圆半径取得越小，则凸轮机构的尺寸就越小，而其压力角就越大。
| 阅读理解。 |
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| Leaves are nature's food factories. Plants take water from the ground through their roots. They take a gas called carbon dioxide (CO2) from the air. Plants use sunlight to turn water and CO2 into glucose. Glucose is a kind of sugar. Plants are glucose as food for energy to grow. The way plants turn water and CO2 into glucose is called photosynthesis. That means "putting together with light". A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is green. It gives plants their green colour. As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter. During winter, there is now enough light or water for photosynthesis. The trees will rest, and live on the food they have stored during the summer. They begin to close their food-making factories. The green chlorophyll disappears from the leaves. Then, we begin to see yellow and orange colours. A few of these colors stay in the leaves all the time. We just can't see them in the summer, because they are covered up by the green chlorophyll. Red and purple colors we see in leaves are made mostly in the autumn. In some trees, like maples (枫树), glucose is stored in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn turn this glucose into red. The brown color of trees like oaks (橡树) is made from wastes left in the leaves. |
| 1. The word "chlorophyll" in the first paragraph means ____ in Chinese. |
| A. 叶绿素 B. 氧气 C. 胡萝卜素 D. 蛋白质 |
| 2. Where is carbon dioxide from? |
| A. Ground. B. Roots. C. Sunlight. D. Air. |
| 3. The way ______ is called photosynthesis. |
| A. plants get carbon dioxide from the air B. plants take water from the ground C. plants turn water and carbon dioxide into glucose D. plants use glucose as food for energy to grow |
| 4. Red and purple colors in leaves are made mostly in ______. |
| A. spring B. autumn C. summer D. winter |
| 5. ______ in the leaves changes into red color in autumn. |
| A. Sunlight B. Water C. Glucose D. Chlorophyll |
