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。