阅读下面短文，并借助上下文内容，在空格里填上一个合适的单词，使短文合理通顺。

阅读下面短文，并借助上下文内容，在空格里填上一个合适的单词，使短文合理通顺。
One cool April 22 morning, people started the day as usual. Yet it was not a normal day. What’s so special about April 22? The answer is simple. It is Earth Day. People all  小题1:   the world celebrate this important day. The idea of having a special day to celebrate Earth began  小题2:     the 1960s. Life was very different then. Few people talked or even thought about the environment. Most people had no idea  小题3:    dirty our air and water were.

On April 22, 1970, twenty million Americans  小题4:    part in the first Earth Day activities. They showed that people really did want to do something about the environment. Since then many countries have passed  小题5:     to protect the air, water, land and animals.
The vehicles such as cars and buses must pass tests to make sure that they don’t cause too  小题6:      pollution. Thanks to the laws, cars today release (排放) about half as many bad chemicals as cars did in 1970. As a  小题7:   , the air is cleaner than it once was.
Rivers and lakes are also cleaner today than before. It’s a good thing. The Cuyahoga River in Ohio was so polluted  小题8:    rubbish and oil on the river even caught  小题9:    ! Today, the river is cleaner. It no longer burns. It’s healthy enough for birds, fish, and other animals.
Now the celebration has spread to other countries. More than 175 countries celebrate it each year. We should  小题10:    our best to help keep our planet healthy.