March 22 is World Water Day. It started in 1993. It not only makes us think about the importance of water, but also calls on(号召)us to 1 and protect water. Today, we're facing terrible water problems. Among them, the wastewater problem is rather 2, And the subject of World Water Day in 2017 is "wastewater".

    What is wastewater? It is used water. Usually, wastewater comes from homes, 3, hospital and so on. It is produced by different kinds of activities, including washing machines, taking showers and using kitchens. The rain also 4 wastewater when it is running down the street during a storm. No matter where it comes from, this kind of water is sure to have 5 harmful in it.

     6 must we treat(处理)wastewater? Wastewater has a big influence in our lives. It causes both illness for us and pollution for the environment. We must care for our environment and our own 7.

    How can we treat wastewater? Different kinds of wastewater need different ways of treatment. Wastewater 8 homes can be reused. Then there will be 9 wastewater. Also, factory wastewater has to be cleaned 10 it goes back to nature.

—They are __________. Picture A is a little smaller than Picture B.