How much rain has fallen on the earth in the past﹖Man has not always kept weather records． Because scientists need a way to learn about past rainfall, they study the tree rings．
A tree's trunk (树干) keeps growing year by year. Under its bark(树皮) , a tree adds a layer（分层） of new wood each year it lives．If you look at a tree stump (树根）, you can see the layers. They are called annual rings (年轮)．
On some trees, all of the rings are of the same width. But the ponderosa (美国黄松) that grow in the Arican southwest have rings in different width. The soil in the southwest is dry．The pines depend on rainfall for water．In a year of good rainfall, they form wide rings．In a dry year they form narrow ones．
Scientists do not have to cut down a pine to study its rings．With a special tool, they remove a narrow piece of wood from the trunk without harming the tree．Then they look at the width of each ring to see how much rain fell in the year it formed．
Some pines are hundreds of years old and have hundreds of rings．These rings form an annual record of past rainfall in the southwest．
66.We could infer from the passage that ＿＿＿＿．
A．a tree grows faster when it has a lot of water
B．scientists cut down trees to study tree rings
C．pine trees form rings of the same width every year
D．the ponderosa grows in the southeast
67.A tree grows a new layer of wood ＿＿＿＿．
A．each week B．whenever it rains C．every year D．every season
68.On the whole, this passage is about ＿＿＿＿．
A.why the tree trunks grow bigger B．why scientists study tree rings
C．trees that lived hundreds of years ago D．the ponderosa in the US
69. The scientists ＿＿to know how much rain fell in a certain area in the past．
A．climbed up the trees B．moved the pine tree
C．removed a piece of wood D．studied the width of the tree rings
70．Which of the following does the passage lead you to believe﹖
A．Young trees have fewer annual rings than old ones．
B．The trunk of a tree never changed in size．
C．Trees in the southwest do not need rain．
D．The soil in the southwest is fit for the growth of trees．