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 |
You can go out, _________you promise to be back before 11 o'clock.
| A．as far as | B．so far as | C．when | D．so long as |
I’m surprised that some of these old houses after the earthquake.
| A．get up | B．stay up | C．put up | D．give up |
For a long time they walked without saying ____ word. Jim was the first to break
____ silence.
| A．the; a | B．a; the | C．a; / | D．the; / |
He had a wonderful childhood, _____ with his mother to all corners of the world.
| A．travel | B．to travel | C．traveling | D．traveled |
It took me a long time before I was able to fully appreciate what they _____ for me.
| A．had done | B．did |
C．would do | D．were doing |