[单项选择]Even before canaries (特高频噪声) were brought into coal mines to alert workers to the presence of poisonous gas, birds were giving us early warning calls signaling the Earth’s deteriorating environmental health. Global bird populations have shrunk by up to 25% since preagricultural (农业社会前的) times. Over the past 300 years, farmland has expanded from six percent of the Earth’s surface to nearly 33%.
Today, three quarters of threatened bird species depend on forests as their principal habitat; each year, however, some 13,000,000 hectares of forests are destroyed, an area the size of Greece. Nearly half the woodlands lost are relatively undisturbed primary forests that are home to a number of sensitive birds and other creatures.
Direct exploitation, including hunting for food and capture for the pet trade, is the second greatest danger after habitat loss, while next is the intentional or accidental introduction of non-native species. As people travel to all parts of the globe, so too
A. Use fences to protect all the parks in the world.
B. Try to protect wild space and nature.
C. Abandon the economic values that birds can bring.
D. Rely less on agriculture and return farmlands to forests.

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