About a third of all common cancers in the United States, China and Britain could be prevented each year if people ate healthier food, drank less alcohol and exercised more, health experts said on Friday. Reports from the World Cancer Research Fund (WCRF) suggest that making simple lifestyle changes could prevent some 40 percent of breast cancers alone in Britain and the United States, as well as tens of thousands of stomach cancers. "Even in 2011, people are dying unnecessarily from cancers that could be prevented through keeping a healthy weight, diet, physical activity and other lifestyle factors," said a WCRF medical and scientific adviser. WCRF findings are supported by World Health Organization (WHO) reports, which say regular exercise can prevent many diseases such as cancers and heart diseases. Cancer is a leading cause of death around the world and its incidence  is rising. Each year around 12.7 million people discover they have cancer and 7.6 million people die from some form of the disease. There are about 200 known types of cancer. Rachel Thompson, the WCRF's head of science, said that while the message was simple -- that not smoking, eating good food and being a healthy weight can help _ many cancers -- it was still a difficult one to get across. The WHO says adults should do at least 150 minutes of exercise a week. This could be done by walking for 30 minutes five times per week or by cycling to work every day. Peter Baldini, head of the World Lung Foundation, also called on all governments to introduce smoke-free laws and raise the price of cigarettes. Tobacco kills millions of smokers every year, and tobacco-related lung cancers also kill hundreds of thousands of people who don't smoke but have been exposed to it second-hand. "There isn't a magic bullet to cure all forms of cancer, but we have the opportunity and the duty to protect people from developing cancer wherever possible," Baldini said. By writing the passage, the author mainly intends to   _  .
advise people to develop healthier lifestyle. warn people of the danger of cancers explain why many people die from cancers every year. introduce the ways to protect people from developing cancers