根据短文内容，判断正误。

    Forests have always been useful and important to men who make use of them in many ways. Every day trees are serving man everywhere. Trees supply man with fruits and building materials in the form of wood. Without trees it would be impossible to build houses, boats, bridges and so on. Furniture such as desks, chairs and beds is made of wood. Trees can stop man from the terrible heat. They're also useful in preventing(阻止) good and rich top soil from being washed away during heavy rains. If there were no forests, heavy rains would wash away the rich surface(表层) soil that is so important to plants. The result is that the land will become a desert. There are plenty of desert areas in the world. A long time ago these desert areas used to be very rich areas, but men in the past had no enough knowledge about science of nature, and they cut down too many trees in the area where they lived and never planted new ones. By and by, the rich surface soil was blown and washed away by strong winds and heavy rains. In the end the rich land changed into useless deserts where nothing could grow.

若$\cos \alpha +\sin \alpha =-\frac{1}{3}$，则$\sin 2\alpha =$（   ）

经济学人智库在报告《长期宏观经济展望：2050重要发展趋势》中预测，以名义国内生产总值计，中国将在2026年赶超美国成为世界最大经济体，并至少将这一领先地位保持至2050年。要成为世界最大经济体，我国必须（   ）