---Was it under the tree ________ you were away posting the letter in the post office?---Sure．But when I got back, it was gone．It's a gift bike from my parents．A.thatB.whereC.whileD.which
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C解析考查从属连词。本句话的意思为：当你在邮局寄信的时候它(自行车)在树下吗?根据回答中的+but when可知两句话强调时间对比,+故选+C。while引导时间状语从句，+意思为“当……的时候”，+从句谓语动词常用进行时。