1. 阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入一个适当的词或使用括号中所给单词的正确形式填空。

    "When it comes to the Earth, there is no time to waste," says the Earth Day Network. And that is right. (protect) our environment is very important. World Earth Day (create) in 1970 to ask more people to protect our environment. On April 22 of each year, people around the world celebrate Earth Day.

    Every year there is a special topic, and in 2016 it was "Trees for the Earth''， Earth Day that year was all about planting trees. They planned to plant 7. 8 billion trees within the next five years.

    That's a big goal, and you might wonder why (tree) are so important. Because they can take in carbon dioxide(CO₂) and improve the air quality(质量). CO₂ is an element(成分) in our atmosphere, but humans make too much of it. person makes about 2.3 tons a year, which is bad for the environment.

    Planting new trees is important because people cut down a huge number of trees. Last year, there were 3 trillion trees in world, about 139 billion in China. But according to China Daily, the world's total number has been almost cut in half since human civilization(文明) began.

That's 15 billion trees are cut down every year.

    So we need (plant) more to make up for (弥补) those we lost. One tree can take in 21. 8 kilograms of CO₂ per year. And it's still one of (cheap) and best ways to get rid of CO₂.

    But don't forget that we can also help protect the environment by (cut) down on the energy(能源) we use every day. Every little thing helps.