题目

短文填词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 【1】 themselves. For example, many trees grow thorns（刺）to tell animals not to go 【2】. Most young trees have large, deep green leaves so that they can catch enough sunlight and change it into their food and 【3】 (energetic). When trees are 4 years old, they begin to grow very fast and become strong enough to 【4】 challenges (挑战) in life. At the age of l5, trees become young adults. They grow more 【5动物获得后天性行为主要与神经系统中的大脑皮层有关．