评估肠蠕动的频率剂程度，听诊时间：（）

A、2min

B、1min

C、0.5min

D、5min

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