When we think of green buildings, we tend to think of new ones-the kind of high-tech, solar-paneled masterpieces that make the covers of architecture magazines. But theUShas more than 100 million existing homes, and it would be __1__ wasteful to tear them all down and __2__ them with greener versions. An enormous amount of energy and resources went into the construction of those houses. And it would take an average of 65 years for the __3__ carbon emissions from a new energy-efficient home to make up for the resources lost by destroying an old one. So in the broadest __4__, the greenest home is the one that has already been built. But at the same time, nearly half of US carbon emissions come from heating, cooling and __5__ our homes, offices and other buildings. "You can't deal with climate change without dealing with existing building," says Richard Moe, the president of the National Trust.
   With some __6__, the oldest homes tend to be the least energy-efficient. Houses built before 1939 use about 50% more energy per square foot than those built after 2000, mainly due to the tiny cracks and gaps that __7__ over time and let in more outside air.
    Fortunately, there are a __8__ number of relatively simple changes that can green older homes, from __9__ ones like Lincoln's Cottage to your own postwar home. And efficiency upgrades (升级) can save more than just the earth; they can help __10__  property owners from rising power costs.

A. accommodations  
B. clumsy   
C. doubtfully  
D. exceptions  
E. expand
F.historic          
G. incredibly 
H. powering  
I. protect      
J. reduced
K. replace          
L. sense     
M. shifted    
N. supplying   
O. vast