阅读理解 There are several advantages in planting trees: they firm the soil, soak up(吸收) extra water and take carbon dioxide out of the atmosphere.  However, it now turns out that planting trees could add to global warming. We all know that tree roots do a great job of keeping soil firmly on the ground and out of the wind's power. The problem is that some of the dust clouds play an important part in soaking up carbon dioxide. Huge dust storms blow out over the oceans from dry parts of North Africa and Central Asia. Tons of dust are lifted and left as a thin film over the ocean surface. Dust from China is carried east and left in the Pacific Ocean. If a treeplanting programme is successful and the dust supply reduced, the next result may be that less carbon dioxide gets locked away in the ocean. Andy, an environmental scientist, has spent the past few years studying dust and says his work shows clearly the complexity of the system. For this reason the need is to focus on cutting carbon dioxide giving off rather than doing anything else. Robert, an American scientist, has shown that when native grassland areas are invaded by trees, carbon is lost from the soil. 'We are studying why the soil carbon disappears, but one theory is that trees do a lot more of their growing above ground compared to grass, so less carbon goes directly into the soil from trees.' said Robert. In wet areas of the world, the gain from trees absorbing carbon dioxide above ground seems to be outweighed by the loss of carbon from the soil below ground. Countries that plan to fight against global warming by planting trees may have to think again. Solutions to environmental problems are often more complex than they first appear, and understanding the Earth's climate is a very great challenge. 1. People usually hold the opinion that________. A. huge dust storms can destroy carbon dioxide B. planting trees could reduce global warming C. huge dust storms can destroy the oceans on the Earth D. planting trees is the only way to control huge dust storms 2. Andy, an environmental scientist, believes that________. A. environmental problems are more complex than expected B. trees shouldn't have been planted in dry places C. dust plays a more important part than trees D. carbon dioxide is harmful to everything 3. Robert's experiment proves that________. A. trees absorb more carbon than grass B. carbon can turn grass into dust C. less carbon can make trees grow faster D. grassland areas should be covered by forests 4. The best title for this passage is '________'. A. The importance of planting trees B. The dust clouds soak up carbon dioxide C. Is it really useful to plant trees to reduce global warming? D. Why the soil carbon disappears?