根据句意，故填trees

【题目】任务型阅读． (每空一词)

Trees are one of the oldest citizens(公民) of our earth. Just like US, trees change a lot when they grow. At one to three years old, young trees learn how to protect themselves. For example, many trees grow thorns(刺) to tell animals not to go near. Most young trees have large, deep green leaves so that they can catch enough sunlight and change it into their food and energy. When trees are 4 years old, they begin to grow very fast and become strong enough to face challenges(挑战) in life. At the age of 15, trees become young adults(成年). They grow more slowly and begin to produce flowers and fruit. It is not until the trees are 20 to 25 years old that they become real adults. The trees reach their largest sizes. Adults trees give US many things such as oxygen(氧气) and natural beauty. If we give them good care, they will go on to live healthily for many years. As time goes on, trees begin to grow older and older and even die. At this time, they still have their important place in nature(大自然). In many ways, the life of trees is like our own life experience. Enjoy every minute of the life of the trees and take care of them!