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.

Laura:Good morning,Tracy. You look different today. 

Tracy: Really? Do I look thinner? I just 38. doing some work on my apartment. 

Laura: I see. What did you do? 

Tracy: I 39. the apartment myself and got a net couch and coffee table for the living room. 

Laura: It sounds like a big job. What colors did you paint your apartment? 

Tracy: Several different ones. Let me see... white, yellow, green, blue and pink. 

 Laura: So that’s why you 40. all those colors on your hair. 

 Tracy: Oh, no!