"Forests are the lungs of our land," said Franklin Delano Roosevelt. Twenty years ago, the world"s lungs were diseased. Roughly half of all the planet"s once-luxuriant tropical forests had been felled and the further deterioration of the Earth"s green spaces seemed【C1】______. Over time countries【C2】______ a "forest transition curve". They start in【C3】______with the land covered in trees. As they get richer, they fell the forest and the curve drops sharply until it reaches a low point when people decide to【C4】______whatever they have left Then the curve rises as reforestation【C5】______. At almost every point along the【C6】______, countries are now doing better deforested are【C7】______down less; reforesters are【C8】______more. This matters to everyone because of the extraordinary【C9】______that tropical forests make to reducing carbon emissions. Trees are carbon【C10】______. If you fell and burn them, you【C11】______ carbon into the atmosphere. If you let them【C12】______they store carbon away in their trunks for centuries. Despite decades of【C13】______, tropical forests are still【C14】______about a fifth of emissions from fossil fuels each year. Encouraging countries to plant trees (or【C15】______ them from logging) is by far the most【C16】______way of reducing greenhouse-gas emissions.【C17】______Brazil had kept on felling trees as rapidly as it was cutting them【C18】______in 2005, it would, by 2013, have put an extra 3.2 billion tonnes of carbon dioxide into the atmosphere. As a way of【C19】______the environment, protecting trees is hard to beat. It is in everyone"s interest to find out which forest policies work—and【C20】______them. 【C10】

A. detectors
B. sinks
C. generators
D. producers