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
B解析这题考查冠词的用法：第一空填an，词组：keep an eye on“密切关注”，第二空不填，是泛指“人类”。选B