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

问题补充：

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

答案：

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。