Many doctors and scientists believe that there are different causes for the different forms of cancer. Working conditions may __51__ cancer. For example, people who work in __52__ contact with chemical compounds like arsenic or asbestos may develop cancer. People who work for long periods of time with X-rays or radioactive material may get leukemia or cancer of the skin, bone or lungs. Smoking is __53__ cause of cancer. Many experiments have shown that smoking can produce lung cancer as well as cancer of the mouth, larynx and esophagus. __54__, viruses may cause cancer. Scientific research has shown that viruses bring about certain kinds of cancer in laboratory animals. Scientists are now trying to find out __55__ true for humans as well. Although we don，t have the answer yet, perhaps in the near future we will know what causes this disease.