问题
选择题
_____ on the subject for half an hour, Liu Ying suddenly stopped. [ ]
A. To be worked
B. To work
C. Having worked
D. Having been worked
答案
答案：C
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_____ on the subject for half an hour, Liu Ying suddenly stopped. [ ]
A. To be worked
B. To work
C. Having worked
D. Having been worked
答案：C