问题
选择题
So between 1510 and 1514 Copernicus worked on it, gradually _____ his theory until he felt it was complete. [ ]
A. to improve
B. to be improving
C. improving
D. improved
答案
答案：C
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So between 1510 and 1514 Copernicus worked on it, gradually _____ his theory until he felt it was complete. [ ]
A. to improve
B. to be improving
C. improving
D. improved
答案：C