Do you want to know something about the history of weather? Don't look at the sky. Don't look for old weather reports. Looking at tree rings is more important. Correct weather reports date back only one century, but some trees can provide an exact record of the weather even further back.     It is natural that a tree would grow best in a climate with plenty of sunlight and rainfall. It is also expected that little sunlight and rainfall would limit the growth of a tree. The change from a favorable to an unfavorable climate can be determined by reading the pattern of rings in a tree trunk. To find the weather of ten years ago, count the rings of a tree trunk from the outside to the inside. If the tenth ring is far from the other rings, then it is certain that plenty of sunny and rainy weather occurred. If the rings are closed together, then the climate was bad for the tree. Studying tree rings is important not only for the history of weather, but also for the history of man. In a region of New Mexico you can find only sand---no trees and no people. However, many centuries ago a large population lived there. They left suddenly. Why? A scientist studied patterns of dead tree rings which had grown there. He decided that the people had to leave because they had cut down all the trees. Trees were necessary to make fires and buildings. So, after the people destroyed the trees, they had to move. In this instance studying tree rings uncovers an exciting fact about the history of man. By studying the dead tree rings, the scientist discovered   _  .
.where the people had left. what the people used to eat how the people left why the people had to leave