单选题 .  All over the world, forests are safeguarding the health of the planet itself. They do this     41    protecting the soil, providing water and     42    the climate. Trees     43    soil to mountainsides. Hills     44    the trees have been felled lose 500 times as     45    soil a year as those with trees.
    Trees catch and     46    rainwater. Their leaves break the impact     47    the rains, robbing them of     48    destructive power. The roots of trees allow the water to go into the soil,     49    gradually releases it to flow down rivers and refill ground-water reserves.     50    there are no trees, the rains run in sheets of water off the land,     51    the soil with them. Land     52    with trees and other plants     53    20 times more rainwater than     54    earth. As they grow, trees absorb carbon dioxide, the main     55    of the "greenhouse effect", which     56    irreversibly to change the world's climate. Together, the world's trees, plants and soils contain three times     57    much carbon as there is in the atmosphere.
    The world's forests contain the vast majority of     58    animal and plant species. The tropical rainforests     59    have well over half of them,     60    they cover only about 6% of the Earth's land surface.41.