It is March 12th. It's Tree Planting Day. There are many people on the hills. They're planting trees. People plant many trees today. Plants are important to us. They can make food from the air, water and sunlight. But animals and people can't do so. Animals and people can get food from plants. Without plants, life will not go on, Plants make the air clean. They can keep water from flowing away(流失). They can stop wind from blowing up sand and earth. But in some places people are cutting down too many trees. This brings about floods and sandstorm. Now more and more people know that plants are important. They will plant more trees and grow more plants. They will try to protect the trees.

我国人口众多，对粮食的需要量很大，因此在许多地区建立了商品粮基地．下列平原中，不属于商品粮基地的是（　　）