问题补充：
They did everything in their power to save the building dating back to the Qing Dynasty, but their efforts were .
A. in turn
B. in time
C. in vain
D. in need
答案：
A. in turn
B. in time
(答案→)C. in vain
D. in need
解析：
试题分析:句意：他们在能力范围内尽一切努力去挽救始于清朝时期的建筑，但是他们的努力是徒劳的。A轮流；B及时；C徒劳；D需要。根据but可知后面和前面内容是转折关系，选C。
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