I. 单项填空：（共10小题，每题1分，满分10分）
1. —Is using a mobile phone harmful to people’s health
—Scientists _____this problem, but no exact conclusion has been drawn yet?
A. have studied B. were studying
C. have been studying D. had been studying
2. —Do you know what water_____?
—Yes, hydrogen and oxygen.
A. consists in B. makes up C. made up of D. consists of
3. Don’t be worried about the small fire in the field, it won’t be long before it_____.
A. burns up B. burns down C. burns out D. burns away
4. When ____ help, one often says, "Thank you." or "it’s kind of you".
A. offering B. to offer C. to be offer D. offered
5. It was _____he came back from Africa that year _____ he met the girl he would like to marry.
