题目内容：
根据以下材料，回答题Why Buy Shade-Grown Coffee?
When people argue about whether coffee is good for health, they're usually thinking of the health of the coffee drinker. Is it food for your heart? Does it increase blood pressure? Does it help you concentrate? However, coffee affects the health of the human population in other ways, too.
Traditionally, coffee bushes were planted under the canopy (树冠) of taller indigenous (土生土长的 ) trees. However, more and more farmers in Latin America are deforesting the land to grow full-sun coffees. At first, this increases production because more coffee bushes can be planted if there aren't any trees. With increased production come increased profits.
Unfortunately, deforesting for coffee production immediately decreases local-wildlife habitat.
Native birds nest and hide from predators (捕食者) in the tall trees and migrating birds rest there.
Furthermore, in the long term, the full-sun method also damages the ecosystem because more chemical fertilizers and pesticides are needed to grow the coffee. The fertilizers and pesticides kill insects that eat coffee plant, but then the birds eat the poisoned insects and also die. The chemicals kill or sicken other animals as well, and can even enter the water that people will eventually drink.
Fortunately, farmers in Central and South America are beginning to grow more coffee bushes in the shade. We can support these farmers by buying coffee with such labels as "shade grown" and"bird friendly". Sure, these varieties might cost a little more. But we're paying for the health of the birds, the land, ourselves, and the planet. I think it's worth it.
What is the main idea of this passage? A．Farmers are changing the way they grow coffee.
B．Coffee is becoming more expensive to produce.
C．Shade-grow coffee is more expensive than sun-grow coffee.
D．People should buy shade-grown coffee.
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