[单项选择]As is well-known, the higher we go above the earth, the ______ air we have.
A) rarer C) rarest
B) rare D) more rare

The earth we live is a ball of very hot rock. It is good for us that the outside shell is cold. If it were not, no one could live here. What if our earth were only as big as an egg Then the outside part would be as deep as the egg’s shell.
The earth’s shell has cracks in it in some places. In New Zealand, this shell seems to have more cracks than in other places on the earth. There are spots where the ground is so hot that it burns the bottom of your shoes. In some places, hot water shoots into the air from time to time. At other points, you can see steam coming up from pools of hot water.
The people who live in North Island, New Zealand, make use of the heat that comes from in- side the earth. Some of them cook in the hot pools. They put food into a basket and place it in the water. Washing clothes is done in much the same way.
Many people bring the hot water from below the earth into their houses, They do this by put- ting a pipe a hu
A. get heat from inside the earth
B. make heat by burning coal, oil or gas
C. live in the Pacific Ocean
D. are too far from the earth heat