C
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(樹干)grows bigger each year.Under its bark(樹皮)，a tree adds a layer of new wood each year it lives.If you look at a tree stump(樹樁)，you can see the layers which are called annual rings.
On some trees，all of the rings are of the same width.But the ponderosa pine(美國(guó)黃松)that grows in the American 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 don't have to cut down a pine tree to study its rings.With a special tool，they can 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.
【語篇解讀】 本文主要講了樹的年輪。
43.We could infer from the story that________.
A.a tree grows faster when it has a lot of water
B.scientists can cut down trees to study tree rings
C.pine trees form wide rings every year
D.the ponderosa pine grows in the southwest
解析 推理判斷題。由第三段第四、五、六句可知答案為A。
答案 A
44.A tree grows a new layer of wood________.
A.each week B.whenever it rains
C.every year D.every season
解析 細(xì)節(jié)理解題。由第二段第一句可得到答案。
答案 C
45.All in all，this story is about________.
A.why the tree trunks grow bigger
B.scientists study tree rings
C.trees that lived hundreds of years ago
D.the ponderosa pine in the southwest
解析 推理判斷題。本文主要講述了樹的年輪和科學(xué)家對(duì)它的研究。
答案 B
46.Why do the scientists study the width of the tree rings?
A.They want to know how big the tree will grow.
B.Scientists want to move the pine trees.
C.The trees depend on rainfall for water.
D.The rings tell them how much rain has fallen.
解析 推理判斷題。見全文最后一句。
答案 D
