碰巧我在办公室有一套备用钥匙。

As it happens, I have a spare set of keys in my office.

《牛津词典》

比如由备用卧室改造而成的家庭办公室，起居室角落里的一张桌子，甚至餐厅的桌子。

It could be a spare bedroom that you've turned into a home office, a desk located in the corner of the living room or even the dining room table.

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我们想建设一个新的总部大楼，但如果花费太多，我们会采取备用计划，只将目前的办公室装修一下。

EXAMPLE: we'd like to construct a new building for our headquarters, but if that proves to be too expensive, we will move to Plan b, which is just to remodel our current offices.

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