By the time my mother came back from work, my sister and I supper and were waiting for he

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A. have cooked　
B. were cooking
(答案→)C. had cooked
D. would cook
解析：考查时态。By表示“到…时为止”，要与完成时连用。本题by后面的时间是一个过去时的时间，故使用过去完成时的时态。句意：到妈妈下班回家的时候，我的姐姐和我已经做好了晚饭在等她吃饭了。说明我们做好晚饭是在妈妈下班之前。故C正确。