In the beach house,there was a small room（）A.being served as

there was a small room （served as a kitchen ）做定语修饰room ,房间和serve（当做.使用,用做）之间是被动关系,所以选B
D选项 in which=where 相当于是副词做状语,
若选d in which served as a kitchen
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状语 谓语 宾语 句子结构不完整,故不能选

答案是C serving as a kitchen。现在分词做定语，修饰room。
B是错的
.. there was a small room (serving as a kitchen). 有一个小房间，充当一间厨房。
这句话可以改成：...there was a small room which serves as a kitchen. 小房间是“充当”厨房
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