Hundreds of years ago, life was much harder than it is today. People didn't have modern machines. Life today has brought new problems. One of the biggest is pollution. Water pollution has made our rivers and lakes dirty. It kills our fish and pollutes our drinking water. Noise pollution makes us talk louder and become angry more easily. Air pollution is the most serious kind of pollution. It's bad to all living things in the world. Polluted air from cars, planes and factories is so thick that it is like a quilt  over a city. This kind of quilt is called smog . Many countries are making rules to fight pollution. Factories must now clean their water before it is thrown away. And they mustn't blow dirty smoke into the air. We need to do many other things. We can put waste things in the dustbin and not throw it on the ground. We can go to work by bus or with our friends in the same car. If there are fewer people driving, there will be less pollution. Factories must clean their water   _  .
before they use it when they use it before it is thrown away whenever they use it