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 becomes highly infectious in __ __humans.

  A. a , /       B. an, /        C. the, an          D. an, the

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 becomes highly infectious in __ __humans.

1. A.
   a , /
2. B.
   an, /
3. C.
   the, an
4. D.
   an, the

 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 becomes highly infectious in ____humans.

      A. a , /        B. an, /       C. the, an       D. an, the