填空题

We all know that trees are very (11) in our daily life. They give us many things, such as wood, oxygen, and many other things. They can also tell us a lot about our (12) , too.
If you cut across a tree, you can see that it has many rings. Most trees grow one new ring (13) . Because of the rings of the tree we know how old it is.
When the climate is (14) or cold, trees do not grow very much and their rings are usually thin. When it is wet and good for trees, the rings are much thicker. If the rings are suddenly (15) or suddenly much thicker, it means that the climate changed suddenly.