[单项选择]The biggest worry that the researchers now have is ______.


A. more and more wild chimpanzees are being slaughtered.
B. AIDS virus is diificult to be killed and controlled.
C. it is not easy to repair people’s deficient immune systems.
D. many dangerous viruses are being transmitted to humans.

[单项选择]The biggest worry that the researchers now have is

[单项选择]

British cancer researchers have found that childhood leukaemia is caused by an infection and clusters of cases around industrial sites are the result of population mixing that increases exposure. The research published in the British Journal of Cancer backs up a 1988 theory that some as yet unidentified infection caused leukaemia--not the environmental factors widely blamed for the disease.
"Childhood leukaemia appears to be an unusual result of a common infection," said Sir Richard Doll, an internationally-known cancer expert who first linked tobacco with lung cancer in 1950. "A virus is the most likely explanation. You would get an increased risk of it if you suddenly put a lot of people from large towns in a rural area, where you might have people who had not been exposed to the infection." Doll was commenting on the new findings by researchers at Newcastle University, which focused on a cluster of leukaemia cases around the Sellafield nuclear repro
A. the cluster of leukaemia cases around the Sellafield nuclear reprocessing part
B. the kind of infection that causes childhood leukaemia
C. the causes of childhood leukaemia
D. a new finding by British scientists

[单项选择]Researchers have studied the connection between aspirin and heart disease for _______.


A. more than a century
B. more than ten years
C. more than one year
D. a few years