阅读下面短文，在空白处填入适当的单词或括号内单词的正确形式。
In 1988, Wang Chengbang retired（退休） from the army. Instead of enjoying a relaxing life, he volunteered to save the soil from desertification （沙漠化） by (plant)trees in Korla, Xinjiang.
It is not an easy job. The high temperature in Korla makes it difficult anyone to work day after day in the sunshine. What's more, water is one of the biggest (problem). In fact, Wang Chengbang's work in the first few years failed very few trees survived（存活）.
But no matter how hard, Wang Chengbang would not give up. "After trying many different ways, I have learned to grow young trees in a better way," said Wang Chenghang. He also worked out new ways (save) water. With his hard work, more and more trees have survived. So far, he has planted more than 1.5 million. Those green trees prevent desertification, make the air cleaner and (clean), prevent people from the hot sun. Now, Korla is a model city in environmental protection.
Wang Chengbang is also (success) in fighting cancer（癌症）. In 2021, he had cancer and (tell) that he had only six months to live. However, he continued taking of his trees rather than lying in bed. "Working is the best medicine. Planting trees helps to fight cancer," he often jokes.
Now although he is at the age of 86, Wang Chengbang is still working.
