单项选择题

Many years ago, life was (41) than it is today. People didn’t have modern machines. There was no modern medicine, (42) .
Life today has brought new problems. One of the biggest is pollution (污染). Water pollution has (43) our rivers and lakes dirty. It kills fish and affects (影响) our drinking water. Noise pollution makes us talk louder and (44) . Air pollution is the most serious kind of pollution. It affects (45) living thing in the world.
Cars, planes and factories all pollute our air every day.
Many countries are making laws (法律) to stop pollution. Factories, must from now on, clean their water (46) away, they mustn’t sent out dirty smoke into the air.
We need to do many other things. We should put waste things in the dustbin (垃圾箱) (47) throwing them on the ground. We can go to work by bus (48) with our friends in the same car. If (49) people driving, there will be (50) pollution.