A recent report has suggested that air pollution in our big cities could be as damaging to our health. The report put forward practical tips that city people can take to reduce their exposure   to city air pollution. Quite a lot, it turns out.  * Watch where you walk  Avoid walking along busy streets and thoroughfares  , instead choosing side streets and parks. because pollution levels can fall by a factor of 10 just by moving a few metres away from the main source of the pollution - exhaust fumes. " Don't walk behind smokers, and walk on the windward, side of the street where exposure to pollutants can be 50% less than on the leewardside.   * Pavement  sense   When you're crossing a road, stand well back from the kerb while you wait for the lights to change. Every metre really does count when you are close to traffic. As the traffic moves, the gases can disappear in just a few seconds, particularly if the wind is up, which means holding your breath during this period can make a difference, silly as that might sound. Besides, cross the road as quickly as possible. And once you're over, continue along the pavement as far away from the kerb as possible.  * Where to sit on the bus  Sitting on the driver's side of a bus can increase your exposure by 10% compared with sitting on the side nearest to the pavement. And sitting upstairs on a double-decker can reduce your exposure too. It's difficult to say whether travelling on an underground train is better or worse than taking the bus, but the air pollution on underground trains tends to be less toxic by weight than that found at street level.  * Wear a mask   Masks can be a good thing, but they only make a difference if they fit tightly and are cleaned regularly. Even the slightest gap to allow you to breathe more easily will  _ any advantages. Worse, if you fail to clean or change the mask regularly, something dangerous can form on the mask. So the air you breathe is dirtier rather than cleaner. Which of the following is the author's opinion according to the passage?
there is no way to escape from air pollution we shouldn't travel on an underground or a bus. we must reduce air pollution to live in the city we must try to protect ourselves from city air pollution.