【题目】All Mrs Smith's efforts to tidy the room were _____ because her grandson soon made a mess of it again.
A. at all costs B. at random
C. in vain D. in return
在线课程【答案】C
【解析】试题分析：考查介词短语辨析A. at all costs不惜代价；B. at random任意地；C. in vain浪费地；D. in return回报；句意：史密斯女士清扫屋子的所有努力白费了因为她的外孙又弄乱了，根据句意可知选C项。
考点 : 考查介词短语辨析