He started to learn English in 1990 and in the years ________ he kept on using it in his work.
Only after the teacher explained it to me a second time _____ the question.
One summer night in 1990, a reporter appeared at the door of Thomas Edison’s laboratory, _____ for an interview.
--- Is Tom still smoking?--- No. By next Saturday he _____ for a whole month without smoking
The traditional goal of science is to discover how things are, not how they ought _____.
In their spare time, they will discuss the _____ to the study of English.