Jerry Morris died on 28 October 2009. He was 99 years old. You have probably never heard of him. He was a professor of public health. More than 50 years ago he produced one of the most famous epidemiological   papers of the 20th century. His study showed that bus conductors were much less likely to die of heart disease than bus drivers. Why? Because the conductors spent their working day walking. It seems obvious now but in the middle of the last century doctors were puzzled by the rising numbers of people who got heart diseases. Jerry Morris found one of the main causes: a sedentary   lifestyle. He started exercising for a few minutes each day and lived until his 100th year. If you wish to protect your heart, you have to do more than wander in the garden. The exercise needs to be reasonable. Jogging is not for everyone and a round trip to the gym takes a couple of hours, plus the monthly membership fee is only good value if you visit regularly. The answer is simple: walk. A half-hour purposeful walk five times a week will lower your risk of heart disease, diabetes and strokes. Older people sometimes feel they have left it too late. But it is never too late to start and there are no upper age limits. Start gently. Take your time: a 15-minute flat walk in the nearest park, four or five times a week. Within a month or so, you are already beginning to protect your heart. Build the walks up. When you can comfortably walk for half an hour in the park, go further: try following rivers and canals. Regular walkers have their own natural gymnasium. There is no membership fee, just some of the finest scenery in the world. Great Britain is the walker's gym. When you have followed the rivers and canals, and are enjoying walking for a couple of hours, head for the coast. Once again, build it up slowly. When you are comfortable with long coastal walks, you can think of our national parks. We can learn from the passage that   _  .
bus conductors are more likely to die of heart disease than bus drivers. doctors in the 1950s knew why heart diseases kept happening to people. walking is better than doing sports in a gym because it saves time and money. British people love walking because they have free gymnasium with finest scenery.