4、We know that trees are useful in our everyday life. They give us many things, such as wood, oxygen, rubber, medicines and many other things. They can _ _ 21 _ _ tell us a lot about our climate. The following are the reasons. If you cut across a tree, you can see that it has many rings. Most trees grow one new ring _ _ 22 _ _ year. Because of the reason, we know how old a tree is. A tree over a hundred years old means that it has more than a hundred _ _ 23 _ _ . When the climate is dry or very cold, the trees do not grow very much and their rings are usually thin. When it is wet and warm, the rings are much thicker. If the rings are suddenly very thin or suddenly very thick, this means that the _ _ 24 _ _ changed suddenly. If we look at the rings on this tree, we can learn about the climate for a hundred years. We can see _ _ 25 _ _ our climate is changing today.