问题
选择题
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
答案：C
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。