题目内容：

    When it comes to green buildings, we tend to think of new ones – the kind of high-tech, solar-paneled masterpieces that make the covers of architecture magazines. 1.They are the witness of the country. And it would be incredibly wasteful to tear them all down and replace them with greener versions. 2.What’s   more, it would take an average of 65 years for the reduced carbon emissions from a new energy-efficient home to make up for the resources lost by destroying an old one. 3.

But there is an unavoidable fact that nearly half of U. S. carbon emissions come from heating, cooling and powering those aged houses. "You can't deal with climate change without dealing with existing buildings," says Richard Moe, the president of the National Trust. With some exceptions, the oldest homes tend to be the least energy- efficient.  4.Houses built before 1939 use about 50% more energy per square foot than those built after    2000.

5.Historic ones like Lincoln's Cottage can be transformed through efficiency upgrades. They can not only help protect property owners from rising power costs; They can save the earth.

A.But how to change their conditions?

B.But the U.S. has more than 100 million existing homes.

C.Some effective measures should be taken by government.

D.So some people say that the greenest way is to maintain old ones.

E.Fortunately, some relatively simple changes can green older homes.

F.Vast energy and resources went into the construction of those houses.

G.The main cause is the cracks that expand over time and let outside air in.