Reading the following passage and fill the blanks.

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 the U.S. has 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 U. S. 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 buildings," 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.