[单项选择]Cancer 1. Cancer, which may arise from any type of cell and in any body tissue, is not a single disease but a large number of diseases classified according to the tissue and type of cell of origin. Several hundred such classes exist, constituting three major subtypes; sarcomas, carcinomas, leukemias and lymphomas. 2. A number of factors produce cancer in a proportion of exposed individuals. Among these factors are heredity, viruses, ionizing(离子)radiation, chemicals, and alterations in the immune system. For a long time these various factors seemed to work in different ways, but now researchers are studying how they might interact. 3. More than 1,350,000 new cases of cancer occur in the United States each year. It is the second leading cause of death in the nation, accounting for about 550,000 deaths annually. The incidence of cancer varies enormously among different geographic areas. The age-adjusted death rate from all cancers in males is 310. 9 per 100,000 in Luxembourg (the hi