Hundreds of years ago, life was much harder than it is today. People didn't have modern machines. There was no modern medicine, either. 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. Cars, planes and factories all pollute our air every day. Sometimes the polluted air 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 flight 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 do not throw them on the ground, there will be less pollution. Rules are not enough. Every person must help to fight pollution. Factories must clean their water   _  .
before they are thrown away when they are thrown away after it is thrown away before it is thrown away
Question: Hundreds of years ago, life was much harder than it is today. People didn't have modern machines. There was no modern medicine, either. 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. Cars, planes and factories all pollute our air every day. Sometimes the polluted air 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 flight 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 do not throw them on the ground, there will be less pollution. Rules are not enough. Every person must help to fight pollution. Factories must clean their water   _  .
A. before they are thrown away
B. when they are thrown away
C. after it is thrown away
D. before it is thrown away
Answer: