1¡¢Last month our English teacher set so difficult an examination problem ________ none of us worked out.

A¡¢that  B¡¢as   C¡¢so that   D¡¢but

2¡¢——Will you go now ?

——Not until I _______ my experiments.

A¡¢have finished           B¡¢finished

C¡¢will have finished    D¡¢had finished