问题补充：
-This problem is very difficult. -I know,but Jack has working it out.
A. succeed in
B. succeeds in
C. succeeding in
D. succeeded in
答案：
A. succeed in
B. succeeds in
C. succeeding in
(答案→)D. succeeded in
解析：句意：这道题非常难。——我知道，但是杰克已经成功的计算出它来了。结合语境可知本句描述的是现在已经完成的动作，故用现在完成时态。选Ｄ。
