Read the passage carefully then choose the correct answers.   

We are all slowly destroying the earth. The seas and rivers are too dirty to swim in. There is so much smoke in the air that it is unhealthy to live in many of the world's cities. In one well-known city, for example, poisonous gases from cars pollute the air so much that traffic policemen have to wear oxygen masks.

We have cut down so many trees that there are now vast areas of wasteland all over the world. As a result, farmers in parts of Africa cannot grow enough to eat. In certain countries in Asia there is too little rice. Moreover, we do not take enough care of the countryside. Wild animals are quickly disappearing. For instance, tigers are rare in India now because we have killed too many for them to survive. However, it isn't enough simply to talk about the problem. We must act now before it is too late to do anything about it. Join us now. Save the Earth. This is too important to ignore.

What do traffic policemen have to do, in one well-known city?