The roots of most terrestrial plants are deeply and widely distributed underground, forming a huge root system, which is more developed than the branches and leaves on the ground. It can be described as "deep roots" and "deep roots of trees". Huge roots can not only absorb a lot of water from the deep soil, but also fix the soil. Using this characteristic of root system, we can keep water and soil, conserve water and prevent soil erosion. The interception of precipitation by trunk canopy is generally 15%. Litter can also intercept precipitation, and forest soil has high water permeability. According to research, in an area with high forest coverage, the low water flow in dry season is about 4 times higher than that in areas without forests, which directly shows that forests can conserve water.

Therefore, choose: B.