【题目】They did everything in their power to save the building dating back to the Qing Dynasty, but their efforts were .
A. in turnB. in time
C. in vainD. in need
在线课程【答案】C
【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。