-----Have you finished your composition already Jack?--Yes I it within half an hour.

问题补充：

-----Have you finished your composition already, Jack? ----Yes, I it within half an hour.

A. have finished

B. finish

C. finished

D. had finished

答案：

A. have finished

B. finish

(答案→)C. finished

D. had finished

解析：考查时态：句意：--杰克，你已经完成作文了吗？--是的，我半个小时就完成了。因为时间是过去的半小时，所以用一般过去式，选C。