下列关于植物的叙述正确的是（  ）

    For hundreds Of years, scientists have been trying to answer what things have an effect on people's emotions (情感). How you feel is controlled by many things. Your emotions can be affected by your personality, by the actions of others and by events around you. Perhaps you don't know your emotions can be influenced by the weather as well.
    People often say they feel good when the sun is shining and sad when it is cloudy. In fact, sunlight makes us feel good. When sunlight hits our skins, our bodies produce a vitamin. Vitamins are natural chemicals (化学物质) which we all need to stay healthy. Sunlight helps our bodies produce vitamin D. Vitamin D helps our bodies make a special chemical. This special chemical affects our brains (大脑) and makes us feel happy. For example, there are long, dark winters of little sunlight in Sweden and Norway, so many people in these countries often feel sad. Without much sunlight, their bodies produce less vitamin D. Without enough vitamin D, their bodies make less of this special chemical which makes them feel good. To help the people in these countries feel better, special “sunrooms” with artificial (人造的) lights have been built. People can go there and get “sunlight” for an hour or two!
    The weather can also affect people's emotions in other ways. Hot weather can make people angry because people feel uncomfortable when they are hot. In the same way, rain can make people angry because being wet can be unpleasant. As you can see, the weather can have an effect on the chemicals which control our brains. Through these ways, the weather can influence our emotions. Sadly, while the weather can change us, we can hardly change the weather.

 是圆的两条弦，  是圆的一条直径且 平分 ，下列结论中不一定正确的是（   ）