There are different ways in which people try to deal with the problem of energy. One way is the
greater production of common energy sources(能源)， such as coal， oil and gas. The trouble with these sources， however， is that they are not renewable.
Another way is energy conservation(节能)， which means using energy more efficiently(有效地).
In some very cold countries people build special houses to save energy. They place materials between the inside and the outside of the walls of the house to keep the cold out and the warmth in. The house is heated by the lights， the body heat of the people and the other equipment in it.
Finally， renewable energy sources are used even though they are often expensive to develop.
One form of these is geothermal energy. In certain parts of the world the temperature of the earth increases thirty degrees centigrade with each kilometre down. At six kilometres， therefore， it rises to nearly two hundred degrees. To get the heat， water is pumped(压；抽)down into the rocks and back up to the surface. Heat from the earth is already used in certain countries.
The writer tells about the "special houses" because they __________

A．show the excellent skills of the builders

B．serve as an example of energy conservation

C．are heated by different sources of energy

D．are warmer than other types of houses

The underlined words geothermal energy in the third paragraph mean________

A．renewable source	B．underground source
C．heat inside the earth	D．temperature of the earth

At a place where the surface temperature is 15℃， how deep do you have to dig so as to get a temperature of 75℃?

A．One km.	B．Two km.
C．Three km.	D．Four km.