In the paragraph that follows this passage, we may expect the author to discuss ______.

A．Radiation has contributed to the disease.
B．Putting a lot of people from rural area in a large towns increases the risk of childhood leukaemia.
C．Population mixing is the most important reason for leukaemia cluster.
D．Childhood leukaemia is caused by an unusual infection.

A．Most people believe childhood leukaemia is due to environmental factors.
B．Population mixing best explains the cause of childhood leukaemia.
C．Radiation has nothing to do with childhood leukaemia.
D．Children born in a large town are at higher risk of leukaemia.