While other scientists are keeping eye on changes in the H5N1 bird flu strain, the scientists in England are working on vaccines, in case it bees highly infectious in humans.
A. a , /
B. an, /
C. the, an
D. an, the
网友回答
A. a , /
(答案→)B. an, /
C. the, an
D. an, the
解析：考查冠词。句意“其它科学家正在密切观察H5N1流感病毒的变化，而英国的科学家正在致力于疫苗的研究，以防止出现病毒演变成对人类具有高传染性的危险。”keep an eye on sth.“注视......”；humans“人类”，前不用冠词。故选B。