_____ the world began,it has been difficult for nations to keep peace with their neighbors.
Although
B. When
C. Until
D. Ever since
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Yesterday morning a prisoner escaped from the prison. It was a long time _____ the guards find out what had happened.
A. before
B. until
C. since
D. when
The sound of the piano became fainter and fainter _____ it was not heard any longer.
A. since
B. after
C. until
D. as soon as
Not until the mother shouted at the top of her voice _____ his head.
A. that the boy turned
B. did the boy turn
C. the boy didn't turn
D. the boy had turned
Although he had not been in the field long, he had succeeded_____other researchers who had worked for years had failed.
A. which
B. that
C. what
D. where