Trees are one of the oldest plants on the earth. Just like us, trees change a lot  1  they grow. At one to three years old, young trees learn to   2   themselves. For example, many tree grow thorns (刺)to tell animals  3  near. Most young trees have large and deep green leaves to  4   enough sunlight and turn it into their food and energy. When trees are four years old, they are  strong enough to  5  the challenge(挑战)in life. At the age of fifteen, they become young adults and grow more   6  . They will have their own flowers and fruit. After the trees are twenty to twenty-five years old, they are called real adults because they reach their  7  . Adults trees give us oxygen and natural beauty. If we  8  them very well, they will live healthily for many years. As time goes on, trees begin older and older and even  9  . At this time, they still have their important places in nature and uses for people.  10  many ways, the life of trees is like our own life experience.

A．before B．when C．after D．since

A．protect B．provide C．prevent D．pollute

A．to go B．to leave C．not to go D．not to leave

A．enjoy B．keep C．stay D．catch

A．get B．receive C．face D．welcome

A．fast B．faster C．slow D．slowly

A．weights B．heights C．styles D．sizes

A．look for B．look after C．look out D．look at

A．die B．hurt C．waste D．throw

A．On B．In C．For D．At