By the time my mother came back from work, my sister and I supper and were waiting for her at the table.
A. have cooked
B. were cooking
C. had cooked
D. would cook
网友回答
A. have cooked
B. were cooking
(答案→)C. had cooked
D. would cook
解析：在英文中用by the time构成的从句修饰主句，主句通常用过去完成时，结构为had+过去分词，故本题选C。