What is the main reason for cancer disease?

Choose the letter A, B, C or D to answer these following questions
There are many causes that lead to water pollution. One main cause of this issue is waste water coming from many factories and then being directly pulled out into water bodies, especially into rivers or seas without any treatment because this is the most convenient way of disposing waste water. Industrial waste consists of some kinds of chemical substances such as sulphur, which is harmful for marine life. Lead is known as the main reason for cancer disease. Cancer has become a popular disease in several communes which are called "cancer villages". Another cause is the awareness of citizens, people always use water for many purposes and then they dump waste water or garbage directly into rivers, canals and ponds and so on. In 2004, because of a bird flu outbreak in Viet Nam, people threw poultry to water body that made water highly polluted.